it really depends on many variables, such as: what kind of media used, how much media used, how many flies it was started with, excelsior or filters. I start mine with more flies than most probably use, because I like a lot of production, but they don't last as long that way so I also use a bit of media. mine are typically starting to dwindle around a month, when I throw them out. they might go for another week or so, but I usually have too many flies, so I discard the old ones. hope this helps, mike.