Most GT's I have seen are in the 10" range. I don't see many beasts that go 12-14" and IF they do go that length, everything I have seen posted on these forums suggests that it takes at least 5 years for them to achieve such a size. If his turns out to be a female, it'll likely 8-10". I would have no issues housing a GT in a 55gallon for life. If it happened to grow to a freakishly larger size however, I would certainly upgrade.. but I'd have a few years on that option from what other monster GT owners have posted.