Hi Claus -
Sail life depends more on how you use the sails, the overall conditions of use, and how you stow them and less on how many times you use them. Storage is especially important because, unfortunately, they spend more time stored than being used.
Sails should ideally be rinsed with fresh water on a regular basis and dried out of the sun. This is especially important if they are exposed to salt water or, depending on pollutant and algae levels, dunked in the lake.
At the end of season they should be cleaned with a good sail cleaning product (see West Marine for sail cleaning stuff) Some products are designed to put life back into the Dacron. There are professional services that actually make older sails almost good as new. Sunscreen and body oils from handling the sails should be removed. Dirt builds up in the weave of cloth and acts as an abrasive to the fibers and wears them out. (This is also true of all your lines, ropes and the stainless rigging. Stainless should be wiped end to end with a cloth saturated with WD-40 at the end of the season. Rope can be washed in a machine - untangling it makes a nice winter evening activity
Sails should not be put away, even temporarily, without letting the tension off the battens. Although some here disagree, I would not roll them up on the boom ever, especially for long term storage.
If they are left up on the boat between trips make sure they are not luffing or snapping excessively in the wind. If that is not possible, drop them.
They should not be put away damp, even remotely damp because mildew is death. They should not be stored where it is too hot, like in the rafters of your garage, although this might make a good temporary place to be sure that they are fully dried.
I think you should be able to get many good seasons out a set used just for racing. A set used just for fun should last many years if cared for. I don't know anyone who changes every two events, but they may be out there.
Have fun, sail hard, sail fast -