That depends on your ethnographical approach:

In classical ethnography, you have very long field phases (often 2-3 years) in order to really get to know your field inductively.

In focussed ethnography, ypur prepare as well as possible, i.e. learn as much about the field in advance as possible and also specify your theoretical question, developing your instruments from that theoretical question. You then have very short field phases (2 weeks - 2 months) and best document your observations with video. The time you "saved" in the field is put in the analysis of the (video) data.

Both approaches have their advantages and disadvantages - basically it is a trade-off between spending time for data collection and data analysis.

If you do not know much about your field, I recommend the classical approach. If you already know much about the field and know exactly what you want to know, I recommend focussed ethnography/video analysis.

If there are several organizations or departments within the organization, I further recommend spending a short time within each department (1 month in the first department, 2-3 weeks in the other departments) in order to get to know your field. The analyse your data and focus your research question (it is easier to see similarities and differences between fields, if one can compare them with data) and then do a second round of field work, this time working more focussedly.

http://www.qualitative-research.net/index.php/fqs/article/view/20/43

http://www.as.tu-berlin.de/fileadmin/i62_astypo3/HK_Schriften_2011-2015/Knoblauch_Schnettler2014-Videography_in_.pdf

Thank so much Nina, you have actually solved my problem. I will also the link you have provide for further information.

The classic criterion for an adequate amount of data collection is saturation. One version is data saturation, which involves the point at which you are no longer gaining new insights from your observations, so that they are merely repetitive. An alternative is theoretical saturation, which is a more complex concept that is primarily used in Grounded Theory.

The obvious problem is that it is not possible to predict in advance how long it will take to reach saturation. If you are pursuing relatively narrow and pre-determined goals (as opposed to exploratory research), then you are likely to reach saturation more quickly.

Thanks you very much David, your explanation is helping me a lot

That depends on your research.

Normally the time required should be enough to close a cycle (a year, a semester, a month... depends on).

In fact, this criterion is which is also known as saturation of information collected (from experience).

Best regards

Thanks Molina

Perfect answer:

"The classic criterion for an adequate amount of data collection is saturation. One version is data saturation, which involves the point at which you are no longer gaining new insights from your observations, so that they are merely repetitive. An alternative is theoretical saturation, which is a more complex concept that is primarily used in Grounded Theory.

The obvious problem is that it is not possible to predict in advance how long it will take to reach saturation. If you are pursuing relatively narrow and pre-determined goals (as opposed to exploratory research), then you are likely to reach saturation more quickly.

What is a reasonable time frame for observation/participation observation in an Organizational Ethnography reseach? - ResearchGate. Available from: https://www.researchgate.net/post/What_is_a_reasonable_time_frame_for_observation_participation_observation_in_an_Organizational_Ethnography_reseach [accessed Aug 3, 2015]."

Grounded theory approach of David read with Girm'a response is the way to go

In order to gain depth of data close attention is required to link your conceptual framework with the methodological approach. Focused ethnographies can be undertaken without using videos for the observations , but always have a risk of being too narrow. This risk can be reduced by having a strong theoretical underpinning within the conceptual framework which can guide your focus and at the same time avoid premature closure. Therefore depth is gained  along with saturation.

Thank you so much guys