The variables are huge for longevity, if you're an animal on the brakes you can do a set of pads in 6k, if you use engine braking everywhere and 'plan ahead' they could last the life of the bike
Seriously though, around 12 - 15k seems to be the 'norm'. I've found EBC 'HH' pads work great in all the big trailies I've had with good feel & power and predictable manners in the wet. There are 'grippier' compounds out there but they wear quick and can leave a lot of dust residue, not normally that clever in the wet either... They are easy enough to change - as long as nothing has seized,
then the fun starts.
