The fatigue life of the fuselage is based on the number of what? Pressurization cycles?
"Aircraft lifespan is established by the manufacturer," explains the Federal Aviation Administration's John Petrakis, "and is usually based on takeoff and landing cycles. The fuselage is most susceptible to fatigue, but the wings are too, especially on short hauls where an aircraft goes through pressurization cycles every day." Aircraft used on longer flights experience fewer pressurization cycles, and can last more than 20 years. "There are 747s out there that are 25 or 30 years old," says Petrakis.
But that only applies to pressurized aircraft, there are large planes that are un-pressurized and their life is generally measured in flight hours. However most large aircraft like say the DC-3 don't have any imposed hourly limit on the fuselage but other parts may be hour/life limited.
The safe/fatigue life of a structure is the number of events during which there is a low probability that the strength will degrade below its design ultimate value due to fatigue cracking. The events may be flights, flight hours, landings, pressure cycles or engine cycles. Safe life may be determined by using a similar structure (usually called a fatigue specimen) which is tested to establish the minimum number of events which should elapse before a major structural failure occurs. For example the safe life of the Cessna 310 wings is 19,190 flying hours.
Metal fatigue is measured in cycles, indeed, but not necessarily in flight cycles. However landing cycles are a good measure of fatigue life, as can be seen in an iron bird set-up at an aircraft manufacturer. Complete cycles are simulated:
A row of actuators are connected to the wing. They start with a bit of rumbling downwards pressure, simulating taxiing.
Then more pronounced rumbling, followed by a sharp wing up deflection, simulating take-off. You can hear the structure groan when this occurs.
A hissing sound gets more and more pronounced: the fuselage is being pumped up like a beach ball. More groaning, it's literally a strain.
When pressure differential is at maximum cruise altitude, we're descending again, after a while we can hear the air escape and the fuselage settle in.
SLAM! A sudden sharp downwards kick on the wing, we've touched down.
And back to the beginning.
These birds make hundreds of flights a day, simulating all that happens during a flight. The wing experiences lots of smaller cycles during taxiing, and two huge ones during TO and landing. It is the number and the magnitude of these cycles that determine fatigue life.
Your question was specifically about fuselages, they are subject to taxiing cycles and pressure increase/decreas cycles. So if we take a decent cross section of runway surfaces and average taxi time, we can tie the taxi cycles to a flight cycle, and continue flying the iron bird again and again...