You don't really need 21T$ for that. 1T$ is enough.
Anyway, dollar-hyperinflation is around the corner. We had hyperinflation of major currencies in the past, and will have it again.
1M$ for BTC might be reached easily this way. Of course this won't make you super-rich :-D
When it will happen? Right when China and other countries stop selling goods and resources to US for fiat paper.
Maybe next year. Maybe 2015.
I really don't think China and other countries will abandon the dollar anytime soon. If they wanted USD inflation, they could just sell all the U.S. treasury securities they hold. The entire reason they buy all these securities is to slow CNY deflation relative to USD. Countries with high trade surpluses with the U.S. would have the most to lose on USD inflation.