3.5 years seems to be the average time, but that is only going by the reports on this forum. I don't think mileage, or how the car is driven plays any part in this failure. I imagine it is the seal perishing. The unit is not very big, so even the tiniest amount of oil getting into the electronic bit would cause issues. Therefore I think the seal is actually degenerating over a period of time, and that is when the seepage begins. I have had a few classic bikes, were previous owners have used cheaper seals and gaskets from non original suppliers. These all perish and either crack or split over time.