Good conventional diesel-electric submarines can remain submerged on battery at slow speed for periods on the order of three to five days. AIP lengthens that. Several AIP schemes in development or already in operation can increase slow-speed endurance to as much as three weeks or a month.
At least for the near future, AIP will be valuable primarily as a low-speed, long-endurance adjunct to the under- water performance of conventional submarines. If their distinctive characteristics are exploited by skillful operators, AIP submarines can be used to telling effect for both short- and medium-range missions. AIP dramatically expands the tactical "trade-space" for diesel-electric submarines. If conditions permit, they can transit rapidly on the surface with-out unduly expending the wherewithal for superior underwater performance. Submerged, they can opt for a long, slow, silent patrol that keeps their batteries fully charged and thus capable of powering speed bursts of significant duration. And by carefully husbanding their resources, they can revert again to slow-speed operation and repeat the cycle several times over weeks of submergence.
Meanwhile, staying underwater longer, without snorting, does pose challenges to the boat and crew e.g. provision of breathable fresh air, phychological state etc. much like you would have on a nuclear powered boat. Submarines are limited by food supplies and the endurance of their crews. Fresh water is made by distilling sea water and oxygen is produced by splitting water (H2O) into H2 and O. The H2 is discharged overboard and the O is stored in tanks for future use. CO2 is removed from the air using an Amine-based CO2 scrubber or by using Lithium canisters (old school method that's still used in emergencies). Any extra gasses like CO or H2 (from the batteries) are turned into water vapor by a CO/H2 burner.
OHio class nuclear submarines go on patrols that last an average of seventy to ninety days .... underwater