DSO (Design Service Objective) is the term I believe you are looking for. The DSO on Boeing models is around 50,000 cycles, but this does vary for different models (i.e. 747's have shorter cycles DSO than 737's). The DSO is just an initial spec for the original design and doesn't mean that as soon as the A/C reaches 50,000 cycles it must cease operation. When you start designing an airplane, you must have some idea on it's longevity. If Boeing were to design an airplane to last a year, then it would be lighter and less costly. On the other hand, if they were to design one that lasts 100 years, it would be impossibly heavy and expensive. The DSO is the same principle that manufacturers of everything from hairdryers, cars, washing machines and televisions use. Fail safe design was the principle used up until about 1978 (for the FAA). In 1978 ( I think) amendment 25.45 of the FAR's revised FAR 25.571 which introduced 'damage tolerance' design philosophy.
This philosophy is based upon the principle that any damage on the aircraft is safe to fly until it reaches a certain size. That size should, in theory, never be reached if the operator performs the MPD (Maintenance Planning Document) mandated inspections required for continuous airworthiness. The MPD contains inspections which will detect any damage before it reaches it's critical size. Therefore, in theory the structure of the aircraft should be able to fly indefinitely if the MPD is adhered to. The only problem arises when the DSO is reached, there is a higher probability of major damage being found as the design wasn't analyzed or tested beyond that life limit.
The reason the damage tolerance philosophy can be used is the significant advances since the early 50's (when fatigue was first discovered and understood). In the 70's, a significant amount of fatigue testing had been carried out so that more accurate crack propagation analysis could be performed . I like to think of the fail safe design as a 'belt and braces' approach, whereas the damage tolerance design is a 'suck it and see' approach.
Hope this helps.