[jedc]

Ohio-class submarines (the ones that carry missiles): when they're "on station" they're just tooling around staying as quiet as they can. There's a relevant phrase for them: "Three knots to nowhere"

Los Angeles and Virginia-class submarines are always doing something: doing exercises, transiting from one location to another, etc. And typically multiple things at once. While the boat is transiting from an exercise area to homeport, the team is doing engineering drills, or other kinds of training. Or the forward part of the boat is doing exercises with a carrier battle group while the engineering team is doing engineering drills. (There's ALWAYS engineering drills or maintenance happening.)

Fuel isn't a primary concern: a nuclear reactor is fueled for the life of the boat, so 30-ish years. That said, effective life of a reactor is something the Navy tracks closely, and depending on the life of the boat, the life left in the reactor, some boats are decommissioned as they get close to the end of their fuel life, and others get re-fueled. (And in the case of the USS San Francisco, who had recently been refueled before it hit an underwater mountain, they cut off the front half of the submarine and welded the front half of a recently-decommissioned submarine on, because the reactor and fuel was too valuable to go to waste)

[ateng]

What about diesel boats? They are exclusively attack subs but fuel is relatively limited.

[jedc]

The US doesn't have diesel boats anymore. Though other countries (like Australia) do.