It depends on the design of the reactor core. They are rates in something called EFPH. Effective full power hours. Normal cruising is about 15-20% of that. At ahead flank you approach 100% and never ever exceed 100%. In port you use shore power and shut the reactor down. My last sub was a 688 and it was pretty much designed to only have one core its entire life. Refueling a Navy Nuclear Power plant especially a submarine is almost as expensive as building a new one. Picture rebuilding your car engine without opening the hood of your car. Oh yeah, and the internal components require massive shielding and safety controls. The actual number of EFPH is classified but the reactor design to achieve it is quite a feat of engineering. Civilian plants have the luxury of lots of space and are routinely partially refueled a couple times a year. But they also run at max output as much as possible.