I typically only make medicated food using LRS or mysis, and consider it "good" for 2 days in the refrigerator. If you freeze, it lasts until freezer burn sets in.
I suppose medicated pellets would last until the liquid turned the pellets to mush.
This is a really good write up by @DexterB on how to mix up medicated food using frozen:The pellets turn to mush pretty much right away.
For some reason, I have a difficult time wrapping my head around how to make medicated food out of frozen food. Does GC + Focus bind to froze food? Presumably, frozen food needs to be thawed before being mixed with medicine. Does one need to drain the liquid before mixing the medication?