Until it died........................Just kidding! A Naso is a very very large Tang. I'd say you could probably keep one for about 2 years before it got to the size where it would really need a larger tank.
My LFS just recently put a full grown 18" long Naso in the L shaped 400g tank and it barely has room to swim. It is currently for sale since the 8 foot long tank is not even big enough for it.