Ive had a pair for a couple years now. They've doubled in size since I got them. Id think that they get to full size in 5-6 years. Someone with more experience should chime in with a real answer though...Im just speculating results based on my own observations.
I will say this though: my o.clowns spent their first 2.5 years in my nano, and grew at a consistent rate. When I transfered them into my 60 gallon, they went through a growth spurt over about 2-3 months. It was amazing to see how quickly they grew in the larger environment.
happy reefin'