I've just replaced my front brake pads at 39000(ish) miles, they weren't worn down to the wear strips, but weren't far off them.
At it's annual service last month they were reported as being 80% worn and needed replacement, which I declined as I already had the pads and it's an easy job, two spanners on the Haynes difficulty scale!
After the first few applications of the pedal, the difference was very noticeable.