The Nimitz class has a service life of 50 years and they are refuelled and overhauled once at their midlife point if 25 years.
Nimitz is planned to be retired in 2022 and replaced by the first of the Ford class, U.S.S. Ford. Only 3 Ford class presently under contract. U.S.S. Ford, currently in sea trials; the U.S.S. JFK, just launched and crewed and being fitted out; and the U.S.S. Enterprise, under construction and due for delivery in 2025.
First carrier they are, currently, considering retiring at the midlife point is the USS Truman in 2024.