First of all rim wear isn't related to miles. It's only related to braking. So you could do 50,000 miles or more of open road riding, yet wear through a rim in less than 10,000 miles of urban commuting. Add rain and sandy roads, or both, and wear life goes down still more.
If your riding profile justifies worrying, start by looking for scoring of the kind that can happen if a stone is lodged in a brake shoe. Some people also place a straightedge again the rim and replace if it's concave by more than 1.5mm or so.
OTOH you can simply be attuned to the signs of impending failure. Most times the rim will flex outward long before it starts to crack, or there may be the beginnings of cracks over a short span. Both of these will cause pulsed uneven braking, so IMO you can wait for that, and deal with it then.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.