The company I work for has 8051's and 80C32's installed and working non-stop in hundreds of thousands of products worldwide for over ten years and have not heard of a MCU core failure yet. At least not where the silicon has worn out. Most, if not all failures are usually attributed to external causes such as excessive heat, ESD, vibration, overstressing, etc.
I would think if you stayed well within the specs and not use the device at or beyond its limits I would suspect your counter/timer will keep on tickin' as long as you don't give it a lickin'!

Having fun,

Hal