From my point view, it depends on the study's objective; if you want to want to measure the knowledge that acquired by participants so the short interval is needed. But if you want to determine when the next educational session or training is needed for patients or staff regarding specific skills. then the duration should be long. So the main key when you can do the post test its your aim. 

At least 3-6 months  if you want to measure the retention

maybe 2-4 weeks is appropriate,ofcourse, it depends on the problem you want to study!

In studies that I am following and according to my esperience, it depends on which area you want to measure.

For knowledge / awareness changes, one month is fine.

Psychosocial assessment for the post-test can be at three and six months.

Skills for one month and three months  

It depends entirely on what construct you are measuring, and how long it takes for that construct to change.  One solution is to start by looking at the means, SDs, and test-retest reliability (without intervention) over that time frame.  Then use that information to calculate a Reliable Change Index, which will tell you whether the change you're seeing would be expected without intervention, given the natural fluctuations in the measure.

http://www.psyctc.org/stats/rcsc1.htm

Is there any literature on the effects of different time intervals between pretest and posttest?

Perhaps I can help.  Got this reference about minimum time lapse for test and retest  using pencil-paper instruments (also applicable to pretest and post-test ) from:

An Introduction to Educational Assessment, Measurement and Evaluation ,2nd Edition (2008), Authors: Gavin Brown, Earl Irving and Peter Keegan..   Pearson Prentice Hall

Rosedale, North Shore, New Zealand (ISBN: 978-1-4425-0069-30

In page 82,  ..  the authors wrote .. concerning time to time comparison (test -retest ) for estimating relaiblity ...

' Do people get the same score on a test at time 1 and time 2? ...  If there is no learning from practice, no recall of responses from the first time and no fatigue from repeating a test, then the true ability should not have changed. Thus scores should be very similar across repeated administration of the test. COMMONLY IF WE REPEAT A TEST SOMEWHERE  BETWEEN 3 TO 6 WEEKS STUDENTS SHOULD NOT HAVE LEARNED MUCH NOR REMEMBERED HOW THEY ANSWERED THE FIRST TIME.  

From my interpretation of the test above, even though, in the duration of 3 to 6 weeks, students would have forgotten nearly all of the pretest questions and answers to replicate the same answers  in the post-test, 

my opinion is,  it is safer ( to defend  yourself in a VIVA)  to  do it after 6 weeks has elapsed . eg. if if you conducted a pretest on the 2nd day of the first week of research, then  the post-test can be done on the 2nd day of the 7th week of research.  This gives researchers using an interventionist /treatment programme a minimum 6-week period to do their intervention/ treatment .

Hope what I am writing is relevant to the your questions. 

Thank you for you 

Dear all,

this discussion related to my research right now. From the discussion above, I could understand that minimum interval time is around 1 month. So is it correct if I say that, if the treatment program in my research is 15 weeks, and that the post test is done during week 16, then it is more than acceptable as the minimum time required is 1 month. Can I say that?

Campbell & Stanley (1963) also suggest that the best time to administer post test is after a month, 6 month and a year of pre test .

in my case, it is 4 months after pre test. So is it valid for me to take minimum 1 month as my justification? Thanks in advance!

depends on the type of training and the aims of this. For example: To analyze learning from the point of view of cognitive learning it is useful to do pre and post test at the beginning of the event and at the end of the event. If you want to investigate opinions and problems, which also serve to guide the event itself, pre-test is done within 15 days of training, in order to be able to analyze it and within 15 days of training, because there is still memory. In my opinion, it is difficult to formalize fixed rules: it depends very much on what you want to analyze, the type of event, the methodology used, and the results you want to analyze (about professionals, patients, the community). The most important thing in my opinion is that there is consistency between the outcome objectives of the event, the methodology used to achieve those goals and hence the short, medium, long term evaluation methodology of the results

do you have journal or any literature to support postest only 1 month?

I am also in need of literature support

Must depend on the time interval between the specification and the need of its update

If I am measuring a gain in knowledge and skills in Social Workers and Social Work Students attending a training at a Nature Center about incorporating sustainability and nature walks into the ways they interact with clients, what is the length of time I can expect to measure with a post test, the actual incorporation of these activities in their shelter? Is two months long enough?

Would I more likely measure a skill or knowledge gain after a pre-test and nature day after two months has passed?

Is what I am looking to measure something I could use a psychosocial assessment tool on?

Thank you for your help everyone :)

P.S. Please send me links with literature to read. :)

P.P.S. My name is truncated because I'm a human trafficking survivor, so when you find out what my name is from my email please do not go share it on social media where I attend college and so on. It's for my safety. Thank you!