I know, it's going to seem annoying, but you simply can't come up with a general thing like that as you can with, say, dogs, because ball pythons and other snakes can be influenced more by external factors during their development than an animal that doesn't develop in an egg.

The only advice I can give you for judging how your snake is doing now is by looking at it. If it doesn't have a "triangular" shape to it and it's looking nice and voluptuous, then you have nothing to worry about in the size department. Weight is a very subjective thing, and the numbers aren't as important as the actual snake itself.