Time between overhauls (abbreviated as TBO or TBOH) is the manufacturer's recommended number of running hours or calendar time before an aircraft engine or other component requires overhaul.
[1] On rotorcraft, many components have recommended or mandatory TBOs, including main rotor blades, tail rotor blades and gearboxes.
They tend toward the lower number if they are new designs, or include boosting options such as a turbocharger.
[2] Depending on the country of registration, aircraft in non-commercial use overhauls may not be mandatory; overhauls at the scheduled times are nevertheless highly recommended for reliability and safety.
Likewise, overhaul at the TBO does not guarantee that the engine will last that long.