It's simple or complex:
1. Time on the water
2. UV exposure
3. Care (where rigged, grass, rocks, sand, etc)
4. Type of use (freestyle, waves, freeriding, racing, etc)
5. Size of sail (I think that larger sails are prone to more wear/damage than smaller sails given equal TOW and other factors).
A sail can last from a half season to 26 years of more.
I still have my Dacron regatta sail and my 1985 Mistral Superlight and the sail looks almost like new.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.