Mileage for this is going to vary greatly based on what kind of riding one does (commuting in the city vs. country back road training rides) and the type of brake pads used. If you are constantly stopping and starting, you'll go through rims quicker. If you barely brake except for when you pull into your driveway at the end of the ride, then you may never wear out your rims.

If you are concerned about tracking wear on the rim, get a micrometer and do monthly/yearly checks depending on how much you ride. Maybe measure how deep the wear indicator is one some other rims and use that a basis for judging how much wear you've got left.