Here is a list of all the picture frames (as far as I know) along with the months they were published. The first PFs were cover dated November and December 1971, although non-PFs were also published during these two months. As far as I can tell, every Marvel comic cover dated 1972 was a PF, although I have highlighted in yellow those that aren't true PFs (in my opinion). After December 1972 the PFs stopped dead with the exception of War is Hell #3, cover dated May 1973.
As far as I know, the last 15 cent issues were published in September 1971. After that the runs either went to 20 cent PFs or 25 cent 52 pagers, some being PFs and some not. I've highlighted the issue numbers of the 52 page PFs in red.
Mike
Edited to include info provided below.