Glad I can help!
Here is some useful info on what is referred to as the D.S.O., or Design Service Objective.http://www.tatsco.com/DSO.pdf
Most airframes are built with a 20 or 30 year service life in mind. Metal breaks down over time as well as cycles. Think of it like a balloon...how many times can you inflate and deflate it before it pops?
The MD-80 has a D.S.O. of 50,000 flight cycles, 50,000 hours and 20 years.
Here is a table from Boeing with Boeing and Douglas types listed:http://www.boeing.com/commercial/aeromagazine/aero_07/corrosn_sb_table01.html
Take my personal favorite, the B747 Classic for example:
The B747 Classic Series (-100 and -200) has a minimum Design Service Objective is 20,000 flights/cycles, 60,000 hours and 20 years.
Now there are exceptions to every rule. TWA retired B747-131 (cn 19676/63) N53110 with 105,759 flight hours
and 19,163 pressurization cycles!