What is the fatigue life of a fuselage based on?
Most (large) aircraft life times are measured in 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.