From my reading of people's comments on tang growth, I think most quickly grow up to around 4-6" depending on the species. After that growth is slower, I believe. This might have something to do with the fact that you need to quickly outgrow predators' mouths in order to have a better chance at survival. Take this tid bit with a hug grain of salt, I am just going off of what I have read as I have no hands on experience with tangs.
I would never put a P. hepatus in a tank that small. IMO, they need at least a 240 gallon tank to be comfortable. There have been several P. hepatus that I have seen in fellow reefers' 4-6ft tanks that just seem to be way too big for the tank, thus making the tank look smaller than it is. Also, some were showing signs of cramped space. Pacing, aggression, destructive, etc. at sizes between 3" and 6".
On a side note, never expect an LFS to take in a fish from you that outgrew your tank. By the time that happens the fish is usually of decent size and much harder to sell than the tiny ones they have. There just aren't a whole lot of people out there that will take in a 6"+ tang.