Depends on the breed. Your standard laying hen can live many, many years (up to 15 I've heard). You won't get many eggs after age 4, but they continue to lay intermittently throughout their life span. Your standard CX meat bird has a very limited life expectancy, rarely living past a year or two. Meat birds are bred to live fast, die young, and leave a beautifully delicious corpse behind. Barring predator attacks, unexpected illness and laying issues, you can expect your chickens to be with you for a long time.