It really depends enormously on terrain and distance is probably a rather more useful scale than time, And it varies front to back too, with the front lasting, for me, less than half the time/distance of the rear. I have to replace front pads about every 3-4 months, but then my typical descent is over 1,500m per 100km and I cycle around 10,000km per year, so that 3-4 months represents 3,000+km and is probably very much low end of pad longevity.