Based on feedback from our customers, the average lifespan is 1 year. Of course it really depends on how much abuse they get, and that's an individual thing.
We used our prototypes 6 days a week for 9 months with TAK before the grip started coming unstitched. Even with some stitching missing, we still used them for another 2 months. We could have used them even longer, but..you know....so many other colors to choose from!