The norm is usually 1500 grams and 2 years of age. This is not set in stone. Some may take much longer, and some can go at around 1200 grams if they are older - like 3-4 years of age).
I bred a 4 year old that was just over 1,200 grams...don't believe I rushed anything at all. I also bred an 18 month old that was 1,800 grams...I may have jumped the gun on her, but she did just fine. I pay far more attention to my snakes' feeding habits and girth than I do weight.