Is there a max time limit for how soon you have to return a frozen stool sample to lab for fresh stool test?

Interesting question: ... And not one that's known to most physicians, i imagine. If no one else is able to chime in, i'd recommend calling the lab you're supposed to submit it to - they should be able to tell you when need to turn it in by.

Depends on the test: Frozen sample would not be suitable for examination of amoeba or other live parasites. Fresh specimen would be needed for that. A frozen sample would be suitable for chemical analysis, and is usually used for 72 hour fat content. For that if the sample is kept at -40 c would be stable for months. It would be prudent to consult the laboratory that will be doing the testing.