On my road bike ('86 centurion) which had an unknown mileage put on the rims before me and probably 4k miles by me I've started wondering about the life of the rims. They don't have a wear groove or wear bar or anything and I really don't know what kind of thickness I would want if I did measure them so I haven't bothered with that yet.

Most of my miles are in dry weather, and when I end up in the rain (doesn't really rain here that much ) I make an effort to rinse off the pads and rim when I get home to minimize "grit wear". I don't see any noticeable dip where the brake pad would've worn away the rim. I can certainly see that it's well worn but not to any extent past the scratching and loss of color on the surface. FWIW they're unmachined Matrix Titan rims.

Is there some sort of standard width considered a safe minimum? Can I safely wait until it appears more worn and there is some sort of visual "dip" from the brake pad? I realize the second one isn't really a straight forward question, but I had to ask.