Depends on how well they're kept, good water quality conditions, temperature, feeding.
Our 6 youngest adults are 12inches + . We got them when they were 3 months old (7cm long) and by the time they were about 9-10months old they had reached 30cm in length.
Our 4 year olds are smaller than them 26-28cm.
They should usually reach their full size, whatever it will be by their 12th or 18th month. Not all axies grow to these lengths. One of our friends also got 4 axies from the same breeder at the same time and his are way smaller than ours (22-25cm) - he feeds them on frozen bloodworm cube and oxheart. Ours are mainly fed on earthworms.