As has been noted, it's always worth a try to clean the seal first.
As for how long they last, that's sort of like asking how soon you'll get grit on the tube - because that's the usual cause for a leak and for seal damage. Sometimes they fail for other reasons, but that's the most frequent - especially if you use your Stelvio off pavement.
I has a fork seal go bad at about 17,000 miles on my Norge - cleaning didn't cure it, so I had it replaced with a third-party product. The bike now has 115,000 miles, and the fork seals are still good. I may replace them at my next major service, just as a preventive measure, since living in the Mojave combines a lot of sand and dust with a lot of heat and UV - all rubber killers.