It honestly depends on the snake. Right now I have a hatchling thats a few months old. I weigh him before every feed. I see changes each time I weigh him.

After about a thousand grams they seem to grow more slowly.

All of this being said, the only real reason I can think of to weigh is if you are uncertain about the amount that you are feeding, or if you are planning to breed and you have snakes that you think are approaching breeding size.