It is not really possible to put a mileage on it as there are far too many variables but leaning on the safe side, a lot of people in the bike industry go off the standard of replace when you wear away .5mm of sidewall. Obviously some rims can go much more than this so it really comes down to balancing risk vs reward.
One other trick some people like to use, although I have never really done it myself is to inflate your tires 10-20 psi over the pressure you normally ride at and let it sit overnight. The theory is, if the sidewall did not blow out overnight you have a little more life in them.
If your riding is usually slow speed and you can have someone pick you up you can run a rim all the way until your brakes pulse but once they start pulsing the end is usually very close. If you tend to ride at higher speeds or in heavy traffic I would play it safer and replace the rims sooner rather than later.
I personally will replace the rims on my bikes that see higher speeds right at the .5mm wear mark, my utilitarian commuter that is mostly ridden at slower speeds and out of traffic I run them until they start to pulse or I can see light cracks. At that point I drop the pressure as low as is comfortable to ride with and ride a different bike until I get the chance to re-lace that wheel.