This posted Boeing document is on corrosion, but asserts that the general design objective is 20 years service life--implying that the cycle objective is derived from that via assumptions regarding route structure and utilization.
It gives design cycle and flight hour objectives by model for many Boeing and (former) Douglas jet transport designs, with the cycle objective ranging from a low of 20,000 for the 707, 747, and MD-11 to a high of 60,000 for the MD-90.
It also observes that many flying aircraft have exceeded their service design life objectives.
service life table