It's really a question that cannot be answered. I would say, generally, if you have good breeder/heritage stock, the birds will lay years longer since they will probably be less prone to egg yolk peritonitis and internal laying. At least, so far, that has been my own experience.
I have lost every one of my original hatchery hens, plus some, to internal laying eyp and/or ovarian carcinoma, however, the remaining original hen is 6 1/2 years old and seems healthy. She hasn't laid an egg in over a year.
I have several hens who are 5 1/2 laying regularly each week, though they all took very long breaks. At this age, they lay, on average, about half to three-quarters of the year.
My 33 hens range from ages of 1 year to 6 1/2, most of them being over 3 years old, and a big portion over 4 years old, and the only one who has completely ceased laying is that oldest hatchery hen. Of those, only three are from the hatchery.
Edited by speckledhen - 5/19/12 at 6:39am