It takes about 6 hours. You can probably expect to leave Tokyo somewhere around midnight and arrive in Nagoya around 6-6:30.
If you don't speak Japanese, Willer Express is probably your best bet. (
http://willerexpress.com/bus/pc/3/top/). If you buy your ticket three weeks in advance, I think you'd be able to find tickets as cheap as 2500 yen.