That would also depend on how often they get bred. If you wait until they are full adults, around 1 1/2 to 2 years, and then alternate breeding every other year it wouldn't be too bad to go as far as 6 or 7 years old, but if you breed a female every year then I agree with Tom 4 maybe 5 years old max.
That is about the answer I was thinking I would get. I had heard of them being bred to about that age and that each breeding takes a permanent toll on their bodies over time.