About how  long will my touchscreen last before it starts to lose sensitivity?
I have a T-Mobile G1 (the first Android phone) that's a few years old now and sensitivity on the touch screen is still intact.

If you don't treat your phone well it is likely that over time you're going to damage it or the screen can get so dirty that sensitivity might decrease, but it is not an issue with the touch screen technology. Any phone that is badly treated will get damaged.