ipod touch 2G/shure headphone problems?

Well here's the problem. I have a pair of shure headphones and i use the foam buds where u squeeze it and it pops back out. Well what i want to know is, how long do i use those for until i should replace them with the ones shure supplies you. I've had the earphones for about 2 months or so and ive been using these foam buds for all that time. I dont want the "maybe few weeks" comment, because a few weeks can be 3 ~ 10 weeks or so and idk. I'd like a specific number such as " 3 ~ 4 weeks". Foam buds feel the most comfortable but when do u think i should replace them with the ones i have boughten. The buds have started to squeeze in and form my ear, and they have gotten wrinkly and the gloss or shine on the bud that they put on it has started to come off. It has gotten pretty dirty and it's starting to build up dust but i wipe that stuff off. Tell me when i should replace it, and how long the buds should last and if i should change it yet.