Well, IMO the real problem is regulating the social structure when you put him in a bigger tank. Trust me, you won't want him to be the first fish in a great big tank. He'll make it his...only his.
As for the amount of time you have until he needs a bigger tank. There are just a ton of variables, and an equal number of opinions on it.
Truthfully, I've seen a scopas tang do well in a 36 for years, and I've seen yellow tangs "pace" themselves to death in 200 gallon tanks.
If it were me I would do it. Start with as small a healthy specimen as you can find. (Just remember, plenty of people plan on having a bigger tank. Many never do. Finding a home for a large yellow tang ain't a sure thing.)