Many months! The biggest limitation is the amount of food they can carry for the crew. The actual limits are classified.
Submarines can produce their own air and water and with nuclear power they could stay submerged indefinitely. The limiting factor is the amount of storage space (there is not much) on a sub for food and the the size of the crew onboard that consumes it. The larger SSGN's can stay submerged longer than the SSN's because they have more storage space.
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Torpedoes are the main armament. older Diesel submarines had deck guns for surface defense and anti-aircraft, but the modern ones do not have fixed deck armament. Of course FBM submarines carry Guided Missiles. More: Most WW2 Submarines carried several torpedoes, and had 4-6 forward firing tubes and 1-2 rear firing tubes. Older subs were not intended to run submerged full time the way modern subs do. There was no snorkel mast (technology taken from captured German subs at end of WW2) on most subs and they had to run on the surface to recharge their batteries for submerged operations. They also were faster on the surface than submerged. So they had surface weapons in addition to torpedo tubes. Usually a main gun, about 5 inches or so; and an anti-air gun similar to a .50 cal. The measurements would vary depending on the country as the US uses standard measurement and most other countries use metric.
In the EPL...3 Subs Universally in the main its also 3 subs
Subs (Submarines).
Everytime one of their nuclear subs sank we had to help them recover lives, etc.; though they often politely declined our offer thru fear that we might see some of their military secrets...which was a fairly valid point.
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