To be honest, in all my years of pedalling, i've never worn through a rim. As alembicbassman has said, keep it clean. It's only when conditions are really muddy/gritty that you have to worry....and it takes many, many miles. TBH alot of the warnings we get on bikes these days are down to the "Caution, may contain hot liquid" warnings on cups of coffee, and "may contain nuts" on a packet of KP.
I used to wear out my MTB rims almost every year here in Lancashire - it's very gritty! I got pretty good at taping a new rim alongside the old, transferring the spokes over then re-trueing.