I did that to an Audi over 20 years ago. I knew the pads were thin, but... When one side finally wore out there was no squeeling, just metal to metal crunch. I drove to my mechanic using the E brakes, wound up buying new rotors too.
On my wifes 2001 A4, I thought I would wait until her pads worn enough so that the warning light comes on. Modern technology! I gave up at 1mm thickness. When I pulled the caliper off, what was left of the brake pad fell to the ground! It was no longer attached to the backing plate. That could have been lots of fun for her in a panic stop!
Moral: Changing pads early is better than waiting. And if your rotors have a large lip on the outer edge, get new rotors at the same time rather than getting them ground down. Rotors and pads are almost as cheap as plugs/wires/and cap, and better for your health.

I have PBR Metal Masters on my V70 which has eliminated almost all brake dust. But, PBR doesn't like to work when it rains and requires more pedal force than OEM. Of course, PBR lasts longer than OEM, but I figure that when the pads are gone the rotors will also need to be replaced at the same time.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)