HI Daniel
Depends what type of backpack you use. You can have the old fashioned ones, which you pump the pressure by hand. Or the modern ones, which come with a battery pack and pump, usually 40/ 45psi.
Some people may rely solely on one as a way of operating a water fed pole. However, others much like me use them, when it is more time saving than running 100 meters of hose around a house.
Usually you could clean one to two hoses based on a 20lt backpack, obviously depending if it was a first clean and how particular you want to be.
I would not recommend this as a sole way to operate wfp, however, it would be cheaper than a trolley system and would be a start for wfp.
Hope this helps
Andrew