Different people will get different mileage on the same type of brake pads. It all depends how you ride it, hard braking and more load/weight are common factors that will make your mileage different from someone else's.
My advice would be to take an odometer reading when you fit some pads and then compare that to when you have to replace them. If you are happy with the type of pads you had on, putting the same ones on again will at least give you some idea of which kind of mileage you can expect from the new pads. That is assuming that other 'variables' (like your braking style/how often you ride with more weight added,etc.) stay roughly the same.
If you knew what the mileage was when you put the current pads on and you know how thick they originally were then you can measure them now and get an estimation of how long they might last (just remember to keep in mind that on your trip you will have extra weight.)
