How long are submarines deployed?

It depends on the type of boat (Fast-Attack, Ballistic Missile, Cruise Missile), the mission, and the state of world affairs.

FBM submarines are on a 3 month rotation schedule, since they have 2 crews for each boat; the boat spends 1 month for crew transfer and maintenance, then time for sea trials, drills, etc., before heading to their patrol area for several weeks.

Fast-Attacks typically don't have a set schedule - they can be gone for Monday-Friday ops, gone for 2 weeks, 2 months, 4 months, etc. It just depends on the boat's mission at the time, be it actual operations, certifications, training, etc.

Global affairs can alter a boat's mission time as well, or inclement weather like hurricanes which might require an emergencie sortie (underway) from port to avoid the storm.

But no matter how long you're gone, there's always something to do.....