When they don't lay enough eggs to pay for their own feed they go into the stew pot. We aim for stew their second fall, around the time their replacements start laying. First year many but small eggs. Second year, fewer but large eggs and stew.
I have a few favorite hens who have genetics I want to propagate. They'll be separated into a breeding flock away from the main layer flock and probably be allowed a few more years to lay, brood and bask in the sun.
In "Day Range Poultry" Andy Lee says he runs Golden Comets for one year then composts the birds. Not much market for stewing hens, not because they ever taste bad, but because they aren't microwavable. There's no cut off point for eating the bird. She'll get tough before a year is out. She'll always make great soup.