what is the average life expectancy of a stock cap and rotor? i have just over 70,000 on it and wonder if its time to think about changing it

 

There is no average. It varies greatly depending on the conditions you drive the car in, how long it sits without use etc.
Daily driven cars seem to get a lot more life out of their opti's milage wise. Scroll down, there a post with someone with 200k+ miles on an opti that's still good.