I agree with Domme Lynx--it depends. How far can a person run? (Of course, I am going to make a running metaphor.) If the domme/caretaker really didn't care about the sub, I dunno, weeks? Do you care about basic hygiene? Bedsores?

Realistically, it depends on the sub's capability to maintain the subspace, and everyone will be different. There is this beautiful point in all heavy bondage sessions where you can see the sub's mind float away, knowing this is where he is, and there is nothing he can do to change it. I imagine this is the sensation you are seeking on extending. When those happy endorphins wear off, you will be bored, then restless, and then it becomes actual torture--not the fun, sexy kind. Everyone has a different "tank of endorphins."

I worked with Mistress Avalon and her submissive, who would easily be "stashed away" for 12 hours (with supervision). I have also seen heavy bondage enthusiasts be wholly spent after an hour. Both are fine.