No risk so far as males go... Just a matter of when they react to female scent and seek to breed. I'd guess in the realm of 30 inches but wouldn't be surprized to find one "mature" at a size less than that. Females are a potential problem in that they yield the largest eggs of any North American colubrid, albeit few of them. The risk then is in becoming eggbound because they are physically too small to pass the eggs out of the body. This is of course a life threatening condition. Based on my own breedings I'd hold out until the female is at nearly four feet in length and carrying significant bodyweight. Good luck...