My longest run was 103 days - continuously submerged - the ONLY reason nuclear fast attack and missile boats surface is to take on food and spares. The two diesel boats in which I served were actually surface vessels that could submerge for a day or two. Given my number of deterrent missile patrols, "northern and ice runs", oddball operations, transit times, and testing - and diesel boat dives - I have just over 6.5 years underwater out of a 26 year (active) Navy career. ~8 years of Shore and Staff duty figures into the surface time.
Look at the video about RICKOVER in my "hour underwater" thread.
Remember that the carrier groups are often deployed overseas for over 6 months at a time.
When USS TRITON completed her 100% submerged circumnavigation of Earth in the '60s that took only 60 days, including some sneaky detours. New London CT to New London CT with no stops.