It depends on food quality, water quality, genetics and the individual itself . One of my axolotls was 1.5cm when I bought him and most of his siblings were that size. The biggest from the same clutch was 6cm.
I've read threads on this forum where some reached full size in 7 months or something. The ones I've raised from 1.5cm are quite big but I still think they have growing to do and they're 13months old now. Like keiko said, 12-18 months is when they start to mature sexually so around that age you would probably expect them to be pretty much full grown.