I think you are looking for a good way to calculate the optimum exposure time to get a given S/N. There are several exposure time calculators (ETC) available on line, including on our site (lcogt.net and search for 'exposure time') and they can maybe be extrapolated for the telescope aperture, filter and source magnitude(s) you are interested in. But there are still imponderables like seeing, or even defocus if the source is bright. So you may need to use an ETC to estimate your exposure time(s), check experimentally to see what S/N you get (compared to what was calculated) and adjust from there. This is hard at first, but should lead to a useful prescription after a few attempts. Good luck.

You probably mean Photography? Any textbook on scientific photography will show you how to do calculate proper exposure time from object brightness (e.g. in  cd/m^2)  and f-stop number (e.g. 1:4) and film/sensor sensitivity (e.g. 500 ISO). For virtually all photographic tasks it makes not too much sense to rely on such calculations since most cameras have reasonable well-working exposure control (also automatic) that photometric calculations do no longer have more than an educational value. If you have a specific photometric problem for which this roundabout assessment is not appropriate, you should give details that would allow me (and others) to give more specific advice. 

Please specify what kind of photometry measurements are you doing and how. CCD photometry of asteroid, exoplanets or any occultation viedeo timings etc. ??

CCD photometry of a star

I recommend to use the Heliocentric Julian Date (HJD) calculated from the UTC of the mid-exposure. There are a lot of tools in the web to do this calculation.

Hi, if I understand your question, and if you want to use CCD captures for occultation timing, from my experience it is best if you "experimentally find a time", depending on your equipment. In occultations, you need optimum ccd signal AND the fastest capture rate possible. 

I think you are looking for a good way to calculate the optimum exposure time to get a given S/N. There are several exposure time calculators (ETC) available on line, including on our site (lcogt.net and search for 'exposure time') and they can maybe be extrapolated for the telescope aperture, filter and source magnitude(s) you are interested in. But there are still imponderables like seeing, or even defocus if the source is bright. So you may need to use an ETC to estimate your exposure time(s), check experimentally to see what S/N you get (compared to what was calculated) and adjust from there. This is hard at first, but should lead to a useful prescription after a few attempts. Good luck.

It depends also on the filter you are going to adopt (I suppose a 'visual range' one), the altitude over the horizon (airmass), and the magnitude of the star. To compute a good ccd photometry, you would have a high enough gaussian-like shape of the star image.  If the star is faint (for your telescope), take care to have a good S/N ratio, if is bright be sure not to saturate it. Take into account also very bright field stars that may oversaturate your image.

I did CCD photometry for many years to build the GPCSII catalogue: the best way to define an exposure time was with trial-error, with initial guess from technical features of your ccd+telescope system.

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If you are using an specific instrument, try looking for a exposure time calculator on the web. There are a lot of these tools available for most of the available instruments. 

Just point your telescope to the sky, and find what you need experimentally. Nothing's better than experience.

It is better to find it experimentally, because the effective S/N ratio depends on observational conditions (light pollution, weather, etc.) .

I recommend as well as Paweł Kankiewicz:

It is better to find it experimentally.

In principle, you can calculate exposure by using the on-line calculators that you can find in most of the web sites of the major telescopes. By the way, the right exposure depends from a lot of parameters such as seeing, position of the field you are interested for respect to the Moon, presence of bright objects near to your target, etc. Thus, the best is to start with the computed exposure, than repeat the observation until you have the best result. Once you obtaind a satisfactory photometry of your object, take a record of the exposure time you used, in order to avoid to waste time next time you will take photometry of the same object or of a similar one in the same sky conditions.

What Vito Grancesco Polcaro said above.  The part about keeping a record of the exposure time you used is important for saving time the next night.  And do check if conditions change--a change in seeing or cloudiness could mean you either over or underexpose if you just assume the first value is correct without checking.

If you have enough time you can test  some  exposure value to find the best SNR (signal to noise) up to seeing and detector efficiency  but sometimes in long exposure you don't have enough time to experimentally find a time.

You should have an idea of the exposure times you need before you observe and check one with a relatively bright standard at the start, as per the comments already made. There is usually a conflict between the S/N you want to achieve and the results the sky quality gives you in the expected exposure time, so it is worth checking that early in the session, in order either to reorganize your exposure times or change your targets to fit the circumstances.

Speaking of memory, what you need the most are standard candles not too distant from your target and comparable filters to what was used to calibrate the standards (or a system you can relate to the standard). You need to have enough time to image both the target and a relevant number of reference stars taken, e.g. in Landolt list. So you need to image correctly your target with the same equipment  & conditions (i.e. meteo) as will be used to image the standards chosen as close as possible. You generally need to have well prepared you night, start before the meridian, be reasonably fast even if the target is faint and end not too far from the meridian before conditions change.

The original publication from Landolt is here:

http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1983AJ.....88..439L

http://www.phy.ohiou.edu/~tss/ASTR410/standards.html

http://www2.keck.hawaii.edu/inst/deimos/landolt_stds.html

Christian Buil's book describes in simple terms how to do the job:

http://www.amazon.com/CCD-Astronomy-Construction-Astronomical-Camera/dp/0943396298

I agree with Eugene Frank Malone. For me it's good to check early in your session the exposure time in order to have some hint about the time you need and then you fine-tune depending on the conditions.

If we aim at best shot, remember that doing astrophotograpy with not perfect weather conditions is useless. Also, better always to take pictures when the target is at meridian, otherwise the airmass will seriously affect the luminosity. If doing photometry, this could happen within the same picture FOV, if far from meridian.

Robert brings up a good point about doing photometry while the target is close to the meridian. If doing photometry of a faint supernova, try to have a long enough exposure such that the SN is at least 10% to 15% brighter than the CCD's noise floor since CCDs tend to be a bit nonlinear at each end of their full well capacity. Ideal exposures of course would be such that the signal is around half of the full well cap of the CCD. This might not be possible for really faint targets. Don't forget about taking additional calibration frames every night. In particular, watch the bias frames as they are being taken each night so that you can see if a new source of electronic noise has cropped up somewhere in the system while taking the new set of bias frames. If the bias frames suddenly become noisier or have an obvious new noise pattern, then you have to find and fix the cause of the new noise (for example, a bad cable connection) and a new complete set of calibration frames must then be taken. Flat fields should have a max brightness which is around half of the full well cap. At least 12 sets of flat, bias and dark frames for calibration. 18 to 24sets is ideal. Anything beyond 48 sets is overkill with virtually no gain. Darks should always be at least a long as your longest anticipated exposure time. If you suspect that the observatory's flat field curtain is not evenly illuminated or there isn't a flat field curtain, then take flat fields 8 hour angles away from the setting or rising sun as the sky is "smoother" at this hour angle. If your photometry targets are faint, then take flats with an average brightness which is only 20% of the full well cap. It is not a bad idea to take sets of flats at 50%, 20%, 10% and even 5% of the full well cap so that you have an appropriate set of flats to use later when calibrating images where the target's recorded signal is very faint to moderately faint. Using the appropriate set of flats later when you calibrate your images helps to compensate for the CCD's non-linearity at the bottom end of its dynamic range if the target's brightness is well below the CCD's full well cap. It is easy to figure out if you need to use a different set of flats when calibrating faint or really faint target images. Adjust the brightness and contrast so that you can readily see your target. If you then see faint ghosting of the applied flat field calibration, then recalibrate the image with the flats which have 20%,10%, or even 5% brightness. You are using the best flats when you no longer see any flat field ghosting across the image background in the calibrated images. With the right flats, the image background should have consistently even brightness across the entire image frame. This then allows you to select reference stars anywhere in the image which have fairly similar brightness as your target to yield more accurate photometry results for very faint targets.

There are a lot of different variables that can influence the exposure (seeing,quality of the sky,instrument,filters and so on) after decades of imaging variables with the same instrument I have no problem at all.I mostly take images in V sometimes,depends on alerts or new object,new filters are requested or if the object is at the limit of my instrument I shoot in Clear,at this point I shoot 3-5 images with different exposure and I check that the SNR is sufficient (should be no less than 20 but I had very good results also with 10)

Depends from many parameters. After a lot of years I can say that the most important thing is ADU level in your CCD for your source: for a good precision, you have to expose till the source reach about of 50% of your ADU scale and a good SNR (signal/noise ratio).