I'm trying to think back, it seems like they molt every month or a bit less when they're little, slowing down to once every couple months >3"
Each molt they gain maybe 10-20% body size if they're been in excellent conditions. Less growth per molt and longer between molts if the conditions aren't as good. As a side note, i had one living with my goldfish for a while, water conditions with goldfish and a human baby thing not being very good, and it was perfectly happy and still grew at decent speed.
I would guess six or seven months from the 1" size to 4.5-5" size, but that is a guess.