Life expectancy of a rim?
Rims can last a very very long time. 

The failure mode (other than from accidents) is simply the wearing-down of the brake surfaces over time (see this question). You can literally wear-away the rim surface with your brakes until the rim is too weak. 

Some rims designed for commuters have a groove in them that is designed to indicate wear. When you can no longer see the groove, you know its time to replace the rim (might as well replace the wheel).

Here's an excellent description of rim wear.