Some I'm sure only look at them as ways to get more snakes and while they don't want their females to die, they don't care if the snake has reached her optimum weight before breeding her.
Keep in mind, optimum weight is a relative thing and 300 grams is a number set by humans, not the snake. I've had snakes that never reached 300 grams (even males who never had breeding stress to slow them down). Fed to capacity, some are just smaller statured snakes. Time to adulthood is also very variable. Depends on the genetics of the snake and your feeding schedule. Some are just monsters. Some seem to never grow despite eating well.
As to age to reach breeding size, we've got a yearling here who weighs over 250 grams. According to her breeder, she's just always been a garbage disposal, and I can attest to that. She'd eat a cow if you threw it in there. I've also got a 4 year old that is only 220 grams. I doubt she'll ever get much bigger.
Over 36 inches, great, over 300 grams beautiful. ...but remember it's an individual thing. Know your snake and their health. Make sure they're well fed, but not obese. If you do have a smaller snake bred, it's best to exercise them so when the time comes, they have some muscle tone to expel the eggs. Alot of egg-binding comes from "cage potatoes" not having the muscle tone to finish the job. When you only have to crawl out from under your hide once weekly to inhale a dead mouse...you don't build up much muscle.
I saw a huge snow a couple shows back. I didn't even ask if it was male or female, but this poor snake looked like a ball python. It was so obese that its skin wrinkled whereever the body bent. It looked stretched to capacity and bloated. If that is a female...I'd bet dollars to donuts she'd egg-bind long before a healthy, active, 200 grammer. I'm not advocating breeding all your snakes at 200 grams, just trying to say that there are other things to look at than just the weight of the snake.
You want a strong snake with good muscle tone and a nice bread-loaf cross-sectional shape. No triangle snakes with their backbones sticking out, no severely obese females. I'd love for all my females to be over 300 grams, but that's not always possible, especially with the smaller kids like some of the Miamis. Flying their meals so they strike and constrict gives them some exercise. Just getting them out and letting them crawl over you helps. Having an exercise cage that you rotate them through so they have new area to explore and branches to climb on helps. As to the weight, if in doubt, it's always better to wait that extra year. If you do go ahead and breed them, keep them active, keep them healthy.
