From the Document: "Complementing earlier research by the Congressional Budget Office, this report examines the availability and use of the Department of Defense's (DoD's) F-35 fighter aircraft. The F-35 has three variants: the F-35A, operated by the Air Force; the F-35B, operated by the Marine Corps; and the F-35C, operated by both the Navy and the Marine Corps. CBO's key findings are as follows:  'Growing Fleets.' DoD currently operates about 450 F-35 aircraft and plans to operate about 2,500 of them by the mid-2040s. [...]  'Aircraft Availability.' The availability of F-35As and F-35Cs declined from 2015 to 2018 but increased in 2019 and 2020. (Availability is measured as the percentage of time an aircraft can be flown in training or on missions.) The availability of F-35Bs has been generally increasing since the aircraft began operating in 2012. [...]  'Depot-Level Maintenance.' Many of the oldest F-35s have spent long periods--totaling a year or more--undergoing depot-level maintenance[.] [...]  'Annual Flying Hours.' Total annual flying hours for each fleet of F-35s have generally increased along with fleet sizes. [...]  'Lifetime Flying Hours.' All three fleets of F-35s are a mix of new aircraft with few flying hours and aircraft with more than 1,000 lifetime flying hours.  'The Effects of the Coronavirus Pandemic.' Although other types of DoD's aircraft experienced increased availability and decreased flying hours during the initial months of the pandemic, there has been no consistent change in either measure for the F-35 fleets since the pandemic began."
Congressional Budget Office: https://www.cbo.gov/