[seafarer]

Most fleet boats were designed for 70-75 days endurance. I remember seeing the figure of 58 days, as one of the longest US patrols of the war. The USS Lionfish's first patrol in 1945 was 51 days (includes 23 submerged), and she never fired a shot.

It seems from reading patrol reports that, barring mechanical problems forcing an early return to port, patrols lasted anywhere from ~40 to ~60 days.

edit: USS Parche's 3rd war patrol was 86 days (10-Sept. to 2-Dec, 1944), but she did make a stop in at Saipan part way through that. She escorted the USS Sailfish in as Sailfish's transmitter was not working. Her 3rd patrol officially ended on 2-Dec when whe returned to Midway for refit.