Simple answer: There really is no set guide. Every snake is different and will grow and gain weight at their own rate. One of my normal girls is 18 months old and still only weighs around 500g, while the pastel I rescued in January 08 at 3 months old and 44g now weighs in at almost 600g.

The best thing to go by is: Is the snake filled-out but not overly plump? Are they active when they should be (i.e. at night or before feeding)? Are they too thin, with a triangular look or prominent spine? Are they regularly eating or are they refusing a lot of meals?
A healthy snake is filled out without being plump, has a round look without sunken in sides or raised spine, eats regularly and gains weight consistently (no matter the amount), defecates normally, has clear eyes and clean scales, and does not drop more than 5-10% in weight during refusal times.
Health is the key, and as long as you have that you shouldn't worry too much about growth rates.