I always change my tires in pairs. The fronts are still good when I do that but not enough that I care to save a few dollars compared to the hassle of going to have them mounted twice.
This time, the front is wore out and there is still some life in the rear. Not a ton, but quite a bit. I am a stickler for tire pressure, check it every ride.
How did I wear the front out first. That has never happened on this bike, or any other bike that I can remember.