It has already been established that most of braking power comes from front brake. Now, several times that I have been replacing brake pads on my commuter cyclocross bike I have noticed that rear pads are worn out but front still have some rubber in them. Does this happen to anyone else, and why would this happen? The brakes are Shimano cantilevers, previously I had no-brand min-V's that did the same.
There are a couple of factors that might come into play.
The first is that you may be braking more than you think at the back. Which brake is controlled by your stronger hand? Are they adjusted the same?
The second factor is dirt. The front wheel throws up dust and muddy water, some of which can reach the back wheel even with mudguards. This can abrade both the pads and the rims. This will be much reduced on disk brakes.
If you use the brakes a lot downhill you may find yourself using the back lightly to stop yourself accelerating. This seems to cause quite a lot of wear, perhaps because the brakes don't remove the dirt like they would in a hard stop and perhaps because it's a lot of wheel revs with the brake touching the rims.
I used to notice this on stock pads and the cheap dual compounds I first replaced them with. The KoolStop pads I now use wear much less and I don't notice a difference. This is on ordinary shimano V brakes. This was despite only taking up the slack in the back brake when coming to a hard stop, but I had a couple of steep downhills.
Generally, front will wear faster, for both the rim and the pads. Bicycle is a bit hard to judge because it depends on the rider's habit, brake pad's quality and surface contact, and also the rim's quality.
I can tell you that front brakes in cars need replacing more often than rear. This example is a bit more objective than bicycle since front and rear are operated by a single brake (although it depends on how the brake is setup and is still subjective).