Depending on the ship, the reactor fuel is replaced every few years.

• Russian icebreakers need to be refueled every 5 years
• The older gen nuclear subs are refueled every 10 years
• Russian Kirov class ships are refueled after around 10-15 years
• US Navy carriers are refueled every 25 years
• Modern nuclear submarines have reactors which do not need to be refueled for the entire life of the sub. i.e the reactors last 30-35 years