The average "HD" movie is less than 2 GB. But Apple is streaming most of this.
My guess is apple stores no more than 100MB per movie or TV show on the device to ensure buffering and playback quality. The fact that is it able to play in seconds of hitting the buy/rent button supports this.
Therefore there could be several gigs of app space left over even with as many as 30 movies and videos rented and ready for viewing.
I'm hoping it'll queue up more than that, maybe the whole thing? My internet connection is spotty, so I like just renting a movie the current AppleTV and letting it download most of the way before watching it.