As far as GM is concerned. the is no maintenance interval. Most mechanics will tell you to wait until it fails and the CEL come on.
My theory is, that by that time, your gas mileage sucks. I like to change the O2 sensor (the front one, if two), when the plugs are changed, just like a good old fashion tune up.
However, if there is/has been any problems that might cause the fuel/air ratio to change, the O2 sensor needs to be changed when the problem is fixed.