You are able to keep a movie as long as you like. If you cancel your subscription to Blockbuster @Home, you will be required to return the disc or pay the un-returned disc fee. The fee would vary depending on the type of disc (ex. DVS, Blu-ray, Video Game).
I sometimes keep Xbox or PS3 games for 2 to 3 months depending on how much free time I have to finish them. Unfortunately, people like me are also the reason the video game wait times are so long. I am on the 3 disk plan and I usually keep 1 disk dedicated to video games and use the other 2 for movies and TV shows.