It varies. I've seen some males not able to breed until they were a good 10" while I've seen both male and females breed at around 3-3.5", and were already successful parents. Usually females are ready to breed as soon as they can lay which is as early as 2.5".
I'd rather go with the age over the size when it comes to males. I'd say if they are over 1 year old, but you gatta worry about one more thing when the time comes............Is the male sterile or fertile?
Hope this helps,
JP