It depends of species, sex, diet, temperature and reproduction. Assuming that you have a Neogonodactylus wennerae from Florida live rock, I can give you the following reall rough estimates from our field data. Sorry, but the measurements are in millimeters. Parentheses give age after settling out from the plankton.
8 mm - 2 weeks (newly settled)
15 mm - 1 month (3 months)
20 mm - 1 months (9 months)
25 mm - 2 months (1 year)
30 mm - 2 months (1.5 years)
40 mm - 3 months (3 years )
50 mm - 3 months (4-5 years)
If you really pour the food to an animal, it may speed up. Females who reproduce don't molt that month. Large females will often have two or three clutches of eggs before taking a month off to molt.
Roy