(V-)Brake pad lifespan Just for the record, my rims are brand new (~800 km) and clean.
I have a set of KoolStop salmon brake pads (actually 2 sets, .... well, front and rear).

I installed them ~600 km ago and I already had to tighten them (adjust the cable tension) twice in the rear, once in the front.
And looking at the pads, the rear are obviously thinner than they were when new and same in the front (although less noticable).

The efficiency is still very good but is that kind of wear normal? what is the usual lifespan of pads like those?

I heard only good about KoolStop stuff,... so don't think it could come from the very pads.
The installation was done properly too, the pads do not touch the rims at all when brakes are released (~2mm away).
It all very much depend on your conditions. If you have a lot of dirt and grime on the road, expect pads to wear out quicker. 

I'd say adjust pads position after 600km is normal, nothing to worry about.