I've found over my 107,000 miles that I have averaged 2 pad replacements to every disc/rotor replacement. The rear has had 3 sets of pads before the disc/rotor had to be replaced.
You should get a warning light indicating that the pad thickness is down low. The saftey squeek sets in shortly after that...about a week. The warning light is tied to just one wheel on each axil, so it's possible a pad can wear too far before the light comes on. I've notice the pads can wear pretty uneven too, which I've never been able to understand why...but sometimes one side will have considerable thickless left while the set on the other wheel are down to nothing.
I would recommend getting them replaced as soon as possible after you hear the squeek. Pads are a real easy do-it-yourselfer too. Rear rotors/discs however are a beast.
You can't really guage the number of miles you'll get out of pad and disc/rotor replacement because it's totally a function of how much you break. Long road trips would skew the number pretty significantly.