I agree with the other posters--breed has a lot to do with it. I don't agree with swapping out hens when the production slows though, as my hens produce less but noticeably larger eggs, so I tend to get as many pounds of egg per bird per year even after they are 3 or 4 or 5.
Just wanted to add that I have a big New Hampshire hen that's 10 this year and was giving me a big brown egg every few day just a few months ago. She quit laying to go broody--but instead of sitting on eggs, she decided to mother the 50 chicks I brought home from the feed store. She might start laying after her motherhood duties are over...she's in good health still, thrashes a 3 year old hen that bullies the chicks at the feed trough, and even gets in my german shepherds face as he walks with me in the henyard, just to let him know not to mess with her babies.
I imagine my Big Red is the exception to the rule, and that most will not lay as long, but they can really surprise you.