90 - 120 hours is the target MTBF for a jet engine? _mind blown_ glad I am not in aeronautics.
There are several different tiers of checks, ranging from short frequent ones, to expensive, time consuming, infrequent ones: http://en.wikipedia.org/wiki/Aircraft_maintenance_checks
Usually, older aircraft that are being phased out are mothballed shortly before their next heavy maintenance check. And the resale value of used aircraft, especially older ones, is heavily influenced by how soon the next D check is.
The MTBF for a military aircraft as a whole system in general would likely be pretty low since it's composed of so many systems. In essence, something is likely degraded on the aircraft at any given time. That is why there are dual redundancies and easily swappable units.