Is this a genuine issue, or is it a marketing scam to get you to need to buy more mascara, thus giving cosmetic companies more money?
I believe its the latter.
Lets look at this bacteria issue for a moment.
1) How would it get inside your mascara? Every tube of mascara I've seen has a tiny opening with a rubber stopper that prevents too much product from getting out, and basically anything else from getting in. You can't put any dirty fingers inside, and you're probably not going to spit in it or blow dirty air inside or something. The only place the applicator wand touches is your eyelashes. I can only speak for myself, but I usually put on eye makeup after washing my face, so theoretically I don't have germs in my eyes when I put my mascara on. On that note, your eyes don't make bacteria all by themselves, and they have a natural way of flushing out anything that isn't suppose to be there (tears!).
2) If it did get bacteria inside, so what? There's bacteria in the air, which gets in your eyes. Its on our fingers, which come into contact with ours eyes and mouths all the time. If you get an eye infection I think its more likely that it'll come from your fingers then your mascara. I got pink eye once when I was 10, and I never used any form of eye make up back then. I haven't had any eye infections since then and my current mascara's are between 2 & 3 years old.
3) Don't we do the exact same thing with lip gloss? We put the same wand on our lips and back into the tube everyday. As long as it doesn't go bad people generally use the same lip gloss for several years or until its used up. If there is bacteria its not going in our eyes, its going in our mouths which to me sound a lot worse. No one's telling us to throw out our lip gloss after 3 months because it might make us sick.
I think its pretty illogical to think your mascara is going to breed some horrible bacteria after 3 months. Like I said above, all of my current mascaras are a few years old and I haven't had any problems. I have yet to have one dry up. I've only in my life used one mascara up and that took a few years and was one with the lash primer on one side & the mascara on the other so it was half what you get in a normal mascara. There has only be one mascara that I thought went bad, but it was 5 or 6 years old and I didn't remember if it smelled bad to begin with or not.
This is just my opinion. You don't have to hold onto your mascara until the end of the world like me, but don't toss out something that still works great and waste that money.
This look here was done with my 3 year old Maybelline Great Lash mascara, it still looks great right? Look no germs...