A new military turbofan fighter engine is not really rebuilt dozens of times, but rather they have:
-test hours,
-teardown inspections, and
- flight test cycles.
Development Teardowns and Inspections:
Initial Qualification: An engine model undergoes a mandatory military specification tests for few hundred hours for endurance etc. after initial build.
Teardowns: After completing these ground endurance runs, which simulate max dry thrust, afterburner use, and rapid throttle etc. Then the prototype is completely torn down for a full teardown inspection.
Rebuilds: Development staff do not repeatedly rebuild a single engine dozens of times. Instead, a pool of 6 to 12 prototype and pre-production test engines are built, running concurrently across ground tests like, in altitude chambers, and flying testbeds. Each specific development engine might be partially or fully torn down and rebuilt 2 to 4 times during the multi-year testing phase to evaluate wear annd tear after going through specific stress limits.
As per industry specs:
Readiness:
Flight Clearance: Before powering a tactical fighter in flight, the design requires thousands of hours of combined bench testing, rig testing, and initial flight-test hours.
Full Maturity: Total development typically demands 1,000 to 3,000+ flight test hours across prototype aircraft before the engine achieves initial operational capability (IOC).