It's possible to play high quality poker for several days without rest. Just takes some building up to it.
Pros often play for ridiculous lengths of time. I recall seeing an interview of Phil Ivey where he was talking about how if he's losing he'll play 40 hours, when the interviewer remarked about this he shrugged it off and said he's happy to go a lot longer than that. Doyle Brunson is another example, in Super System he refers to having played a 5 day game without more than a 5 minute break (having meals delivered to him at the table).
Even the small time pros tend to put in long sessions. Most of the pros I've met tend to have no issues with playing for 24 hours.
Personally I'm by no means as skilled as a professional poker player is assumed to be but I do make a modest living off of it, and preffer to play 24-30 hour sessions simply because I don't like quitting when I'm losing (becoming determined to overcome bad beats), or quitting when I'm winning (if I'm making money so easily, I should stay at the table until the game becomes bad).
Long sessions also have the advantage of requiring fewer days per week of "work", which allow you to more days per week to spend on the rest of your life (at the cost of sleep deprivation.... ).
I really do believe that for the first 24 hours your play wont suffer if you just do as much as you're comfortable with, until you can easily do the 24 hours (it's more a question of dealing with the lack of sleep than anything else). The next 24 hours can get a bit ugly, but I think you could probably build up to that too if you wanted (although I question how healthy regular 48+ hour sessions would be... 48 hours will mess you up quite a bit).
Most people wont have any need to approach these session lengths because they're just playing for kicks instead of a pay check, in this case then as much hours as you can find for poker in which you'd like to play poker is enough. No alterations to that quantity need to be made.