Rim wear because of braking is usually the limiting factor. If you ride a lot in the wet they will wear faster because the grit from the road gets ground into an abrasive paste.
Many rims have wear indicators, usually indentations in the rim surface. When they have disappeared, the rim is finished. If they aren't present, you can still tell if a rim is wrn by examining the braking surface. If it is badly worn you can usually feel that it has become concave.