Q: How long are roosters fertile?
A: Like with any animal, this can depend on the particular bird. That said, roosters will probably be fertile most of their lives---and if you have a great rooster, he's half the flock, so it makes sense to allow him to fertilize eggs for as long as possible to pass on those good genes.
Black Frizzle Bantam Rooster
If you're worried, just check your eggs to see how their fertility is. If they are mostly fertile, your rooster is doing his job!