From what the experts are saying who were involved in this deal, or who know about subs, nuclear subs can stay underwater for as long as the crew can stay fed and watered!
Just googled it. "The limits on how long they can stay underwater are food and supplies. Submarines generally stock a 90-day supply of food, so they can spend three months underwater. The diesel-powered submarines (not now used by the United States Navy) had a limit of several days submerged."
The non nuclear subs, from what I've heard, maybe it's the current Australian ones?, need to come to the surface all the time to recharge their batteries so it leaves them vulnerable to detection and possible attack during times of war!
