Here's something on the topic. http://bike-works.blogspot.com/2009/...-wear-out.html

There's really no way to say how long a particular rim will last because it's so dependent on conditions and amount of use. If there's a wear indicator groove cut into the rim that is one way to tell. When the rim wears down to the groove you should replace the rim. Other than that I would suggest cleaning off and visually inspecting the rims. Look at the braking surface and see if it seems badly worn. Maybe someone else will have some advice. Why are you concerned about it?