Yep, I have been a subscriber for a while and you can keep the same disk the entire time you have an account if you want. It would be cheaper just to buy the movie at that point though. They don't care how long you keep it as long as you are still a subscriber. It actually saves them money if you keep movies longer because they don't have to pay the costs of shipping movies to you as often.
This also applies to movies you pick up through the unlimited in store exchanges at your local Blockbuster store. You don't have any due dates like the other customers who rent from the store so you are free to keep them as long as you want.