For the female I think it's best to wait until she's about 3 years old. If she's at least 1,2 m long, you can breed her sooner than that, but I think it's better to give her the time she needs to grow up. In the long run, you'll end up with a bigger, and therefore more productive female. I know Black Ratsnakes have been bred much younger than that and much smaller than that, but I do not recommend this. For the male it really doesn't matter that much, it won't cost him a lot of energy so it will not slow down it's growth significantly. Most male Black Ratsnakes are ready to breed at 2 years of age, if properly fed that is. For best results I recommend brumating your animals for 3 - 4 months at 10 - 15 degrees Celsius.