It generally depends on how much use they get. I think with daily use, they can be used for 1-3 months. I guess it also depends on how OCD you get about their condition. I've seen some people use foam tips that were falling apart, whereas other people may want change them as soon as the foam loses it's elasticity. Also, different foam tips can last longer than others. For instance, Shure olives are said to last longer than Comply foams.
I've been using the same pair of comply foams for a few years. Not every day, but enough that it's relatively worn though not falling apart. Maybe they sound worse than new ones? probably up to you. I have a bunch of new foams as well and have tried them, but can't hear much of a difference so I haven't bothered replacing yet.