If we are talking hybrids, then any eggs after 2 years old are a bonus, but some, like the black rocks can still lay up to 5 years old.
Pure breeds will usually lay for longer but they generally have a rest during the winter.
Saying that though, a young man came here the other day to buy some chicks, and he was saying that his friend has a French bantam, but he doesn't know that breed, that still lays occasionally and she is 17!!! years, not months!!