It's an average. I can grind my rims one time in 2 weeks or 2 times in two rides. It depends of the weather conditions, the time you ride and if you have comp. I often ride, and when I have comps I automatically grind my rims.
Ride in the rain in a place where there is some sand, dust, ..., and you will see that your grind will hold only the time of one ride.
But here is not the question, so does anybody have another solution ?