I think the answer to that question really depends on what breed your chickens are and how well you care for them. In my opinion, production breeds (think Leghorns and such) seem to burn out faster then dual purpose/heritage breeds. For example, my friend keeps white leghorns, she replaces them every 2 years like clockwork because their production slows down. My Chentechlers are a dual purpose/heritage breed, I have one hen that's 6 or 7 years old and she still gives me a couple of (ugly, wrinkled) eggs a week if the weather suits her. That's the way I look at it at least. Good luck!
Hens are born with all the eggs they'll ever lay. Some breeds have been bred for egg production and are genetically programmed to be "egg-laying machines", laying an egg almost every day for months at a time. The heritage breeds on the other hand might only lay 3-4 eggs a week at their peak. Neither is born with more eggs than the other - but the production bird will lay all of her eggs in a much shorter time than the heritage bird.
Add in other factors - like whether you have lights on them over the winter to encourage them to keep laying, or let them take a natural hiatus over the winter months - and that will make a difference too.
I have a 3-year-old who still lays 2-3 eggs a week. She does a major molt every Fall and stops laying for a good 8 weeks, which has enabled her to keep laying longer than if she had continued to lay during those times.
As for how long they live.....I've heard they can live 12-15 years. In reality though, because almost everything predates on chickens - from domestic dogs to bears, raptors to snakes - few live that long.
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.
Thanks so much, I have learned so much in such a short time. My Eldest is a mostly white with some orange tinges and I was told she was a Rhode Island Red, I have one more with more orange/red and 2 Barred Rock. Everyone but my eldest lays 1 egg a day usually. They all free range throughout the day and coop us at night.
I have had the egg qualitity also decrease as they age. I like thick whites for fried eggs. I have read on here that there are some birds that live long lives, but I think it sets you up for disappointment. If you bought your birds at a feed store, I would think a reasonable age is 3 years old. I have had a lot of birds die around 3-4 years, but I have lost a lot of birds to predators too.
To me, anything beyond 3 is bonus. I an not religious about moving them out at 2 or 3 years, but I add a few, and loose a few each year.