From what I know it does 2 things:
1. You rent a movie/show and once you play it, you have 24 hours to view it.
2. You buy a movie/show. You can then either download it to your computer and watch via homesharing, or you can go into Movies and select Purchased and that will stream it to your Apple TV without needing to download it.
I've done both before and it shouldn't take hours. Is this a recent issue for you, have you been able to stream a movie before without issues? Or has it never worked for you?