Longevity in production is something we should all be striving for. I am building my line of LF Brown Reds around a hen that is coming up on four years old and still one of my best layers, though she is going through a molt right now. The two cockerels I saved for breeding this year are both out of her. She was never a show bird because of coloring issues, but around here they have to produce in order to stick around.
Unless you are breeding for it, I would not expect more than about two years of good production. It does help if you feed so they don't get too fat, and to keep them free of parasites on a regular basis.