What is the lifespan of an airline engine?
In general, the engine life is measured in terms of two things:


Flight hours
Flight cycles


A takeoff and a landing counts as a flight cycle. 

The most important concept in engine life is the Time Between Overhaul(TBO), which is basically the manufacturer recommended time period after which the engine is stripped down,checked thoroughly and required parts replaced. After overhaul, the engine is usually cleared till the next TBO. 

An overhauled engine is theoretically as good as a new one and comes with the same life (TBO) and warranty, if applicable. Airline engines (for example the Rolls Royce Trent series) usually have TBOs of over 15000 hours. The record for maximum time for an engine on wing (i.e. use in aircraft before removal for overhaul) is well over 40,000 hours.

The life of an engine is given by the manufacturer and are different for different engines. However, the engine life depends on a number of parameters and is usually different for various engine components. 


The rotating components have life limits which are less than the engine and are replaced periodically, mostly during the TBO.
For example, the turbine blades are replaced after a set number of turbine cycles and/or after a set number of hours, which are lesser than the others.
In addition, most rotating components (compressors, turbines) are periodically subjected to checks, and are replaced if found damaged.
In case of abnormal operations, some parts are required to be replaced. For example, if one engine is required to give excessive thrust (for example in case of One Engine Inoperative), the engine is subjected to thorough checks similar to TBO.


Airline engines are usually serviced (Overhauled at TBOs) and maintained till they are deemed not good to use or not economical to (service/repair and) operate anymore. 

Most of the engine manufacturers employ condition based health monitoring programs, which helps them to plan when to service and repair the engine. As a result, the engine life varies based on its condition.