Hi Zac,
A lot of factors would determine this, but my 50+ juveniles began morphing a year ago. The others are a couple of months younger. Some of them are about 4 - 4.5 cm at this time. Some eat better than others. I try to regroup them based on size every so often, so the smaller ones have the same chance to hunt their prey as the larger, more aggressive ones.
Mine do not seem ready to return to the water, however. I wouldn't consider them "adults" until they are aquatic again.
Hope this helps,
Dana