When I was growing up, I learned that the American Dental Association recommended replacing your toothbrush every three months. I don't think I ever religiously adhered to this guideline, but I do think I unnecessarily retired quite a few toothbrushes because of it. I always thought it was odd that my dentist only gave me one toothbrush every 6 months, though. If changing your toothbrush was so important, why didn't he give me two? I'm sure toothbrushes are very cheap, if not free, for dentists.
On the other end of the spectrum are those people (and you've seen their toothbrushes) who seem to use their toothbrushes for ten years. The bristles are splayed out everywhere like an automatic car wash brush. At this point, the toothbrush has lost its shape and probably most of its effectiveness.
I have been using the same toothbrush for the last year, and it looks very much the same as it did when I started using it and still works just fine. I'm inclined to think that you don't really need to replace your toothbrush on any specific schedule--that's just the industry line to get us to buy more. As long as your toothbrush still looks similar to how it looked when you bought it, I say keep using it. Just make sure you clean it once in a while, for cosmetic reasons, and if you're worried about germs, dunk it in some boiling water every few months. And when you're finally ready to retire a toothbrush, save it to clean difficult parts of the shower. I find toothbrushes to be better than larger brushes for cleaning grout and shower window ledges.