A traditional way to measure time in plants is the "Plastochron Index".  The notion is rather like measuring age in humans by counting birthdays.  If you say you are 30 years old, for instance, what you really mean is that you are 30 years, + nine months, + some fraction of the current year.  Similarly, in plants you can count the number of leaves on the stem when some event occurs, but you cannot count the number of tiny leaf primordia enclosed in the apical bud which are too small to see.  So you count the number of leaves larger than some "index size" which is large enough to measure but still while the leaf is growing.  (I'm scratching my head here trying to remember how this goes).  But the process depends upon the the leaves being produced on the meristem at regular intervals, and that the chosen index size is measurable but also that the leaf is growing exponentially.  If these conditions are met then leaf size can be plotted linearly on semi-log paper and the true age of the plant can be determined in "Plastochrons". 

The idea is that our "clock time" is not necessarily the best measure of time and age for a plant.  The plastochron index gives a reasonable measure of age and time that relates directly to the plant in question.

The original concept was laid out by  Erickson R. O.,Michelini F. J.. 1957. The plastochron index. American Journal of Botany 44: 297–305.  More recently it was reviewed by Lamoreaux R. J., Chaney W. R., Brown K. M. 1978. The plastochron index: A review after two decades of use. American Journal of Botany 65: 586–593.

A much more recent presentation which I have not read can be found in  Am. J. Bot. July 2009 vol. 96 no. 7 1313-1318, Chen et al, A new method to measure leaf age: Leaf measuring-interval index

One standard used for Arabidopsis and other species is the time (days after planting, for annual species) or the date (in the year, for perennial species) of the first flower opening (bud burst). 

There are several ways:

1)Record the number of days since initial planting to the first flower,  or

2) From the emergence of the first cotyledon to the first flower, or

3) for many plants (NIL, F3, etc.) - from  initial planting/emergence of the first cotyledon to the flowering of 50% of plants 

Record the number of flowers and buds on  the plants. 

Useful:

http://www.ncbi.nlm.nih.gov/pubmed/20734264

http://www.ncbi.nlm.nih.gov/pubmed/10649289

Wood, A.J. and Roper, J. (2000, March). "A Simple and Nondestructive Technique for Measuring Plant Growth and Development." American Biology Teacher, v62 n3 p215-17.

Counting leaf number, after they have flowered. For Arabidopsis, it is the standard measure for flowering time. 

A traditional way to measure time in plants is the "Plastochron Index".  The notion is rather like measuring age in humans by counting birthdays.  If you say you are 30 years old, for instance, what you really mean is that you are 30 years, + nine months, + some fraction of the current year.  Similarly, in plants you can count the number of leaves on the stem when some event occurs, but you cannot count the number of tiny leaf primordia enclosed in the apical bud which are too small to see.  So you count the number of leaves larger than some "index size" which is large enough to measure but still while the leaf is growing.  (I'm scratching my head here trying to remember how this goes).  But the process depends upon the the leaves being produced on the meristem at regular intervals, and that the chosen index size is measurable but also that the leaf is growing exponentially.  If these conditions are met then leaf size can be plotted linearly on semi-log paper and the true age of the plant can be determined in "Plastochrons". 

The idea is that our "clock time" is not necessarily the best measure of time and age for a plant.  The plastochron index gives a reasonable measure of age and time that relates directly to the plant in question.

The original concept was laid out by  Erickson R. O.,Michelini F. J.. 1957. The plastochron index. American Journal of Botany 44: 297–305.  More recently it was reviewed by Lamoreaux R. J., Chaney W. R., Brown K. M. 1978. The plastochron index: A review after two decades of use. American Journal of Botany 65: 586–593.

A much more recent presentation which I have not read can be found in  Am. J. Bot. July 2009 vol. 96 no. 7 1313-1318, Chen et al, A new method to measure leaf age: Leaf measuring-interval index

You already got good advice for arabidopsis. Actually, the answer depends on the species of your interest, and on the experimental setup. I mean, it is different to record flowering time for single plants grown in pots or at field plots of crop species. Species is also important, as floral morphology and development varies among them. Many have well established protocols for flowering time recording.  Is there any other species of your interest, besides arabidopsis?

Number of leaves before flowering is standard for flowering time in Arabidopsis, pepper, tomato and others.

There are two divisions of crop scheduling which require the "start" time to be marked in different ways. Those starting from germination eg tomato and those starting from some other event such as "stopping" or "pinching" where apical dominance is released by the removal of actively growing material above an apical meristem. The rate of leaf initiation of such an inhibited axillary meristem will be very slow. A destructive sample can be taken to count this starting leaf number before "pinching" in eg chrysanthemum or flower cutting in eg rose. After cutting off the dominant influence leaf initiation rate will become much more rapid. A further series of destructive samples then need to be taken to determine an estimate of flowering time and leaf number. Plastochrons can be determined from this data and they can be expected to vary according the stage of development the vegetative shoot  is at as it progresses towards the flowering event. In a sense germination is a release from dormancy in a way similar to the dormancy maintained by apical dominance over an axillary shoot. Maybe these SEMs would help to illustrate this: http://www.freewebs.com/horridge/

http://www.freewebs.com/horridge/

I am looking to record this parameter for Blueberries

I'm not too familiar with blueberry. This wikipedia photo:

 http://en.wikipedia.org/wiki/Blueberry#mediaviewer/File:Vaccinium_corymbosum0.jpg

gives the impression the inflorescence of about 28 or more individual flowers shown is a terminal inflorescence which might have started out as an axillary bud. This means that the number of leaves initiated by this axillary bud can be counted as the leaf number to flower.

Thanks A lot Jon !

I want to measure days to 50% flowering in rapeseed grown in pots having three replicates. Any useful suggestion please?

Hi Muhammad,

One standard measurement for rapeseed flowering is days until the first flower opens in half of the plants. Here is a reference using that method

https://academic.oup.com/jxb/article/62/15/5641/562188#83490763

hope this is helpful

If the link doesn't work here is the title of the publication

Flowering time variation in oilseed rape (Brassica napus L.) is associated with allelic variation in the FRIGIDA homologue BnaA.FRI.a