Celticgirl, rims wear out from caliper brakes over time. Fact of life. It's part of the stopping process. (Discs do too.) How fast depends on a lot of factors mentioned above. Riding in the wet speeds up the wear a lot. Likewise riding in abrasive dust, sand and gravel. (Now, not all dust, sands and gravels are alike. There are places famous for really abrasive rock, sand and dust. Where I live and ride, the Pacific northwest, is one. All of us here know we go through rims at a rate we are frequently told is impossible by those in other areas. I kill rims in less than two winters commuting 60 miles/week.
Also as said above, with a well built wheel and an identical (at least for ERD, effective spoke diameter) replacement rim, swapping the rim over and reusing the spokes is very do-able. I go 3 rims per set of spokes. I don't know what you are riding now and if they qualify as a "good build". A good wheelbuilder could assess that. But if rim wear is real for you and your riding, have your first re-rim done right, by someone good who knows you are going to use his spokes on the next rim.
The simple test for rim wear - place a straight edge across the rim. (A credit card will work as well as anything.) Look at the hollow. I can't give you hard numbers to go by, but if small bugs can get through, replace it. (Rim walls vary in thickness and I don't have easy access to those numbers. I have seen rims that have been ridden too long. Don't! It isn't pretty. Both you and your bike could see significant damage if the sidewall blows off. If you see lengthwise crack or even the merest start of a hole along the middle of the rim wall, don't ride another inch and let the air out of the tire.)
Many rims have a groove along the sidewall. Look now for that groove. It is a a wear indicator. When you now longer see it, you've arrived at replacement time. (No, it won't blow up tomorrow, but that is your service reminder.) If you go the wheelbuilder route, seriously consider buying replacement rims at the same time. Then, 4 years from now, you go back to him (or someone else) with your dusty brand-new rims in hand and pay him for just labor. No risk that rim is now unavailable and new, different length spokes must be used.
Ben