The darts on the 748 use about 5gal/side/takeoff (of the water/meth) if it is used "conservatively". That is run "wet" until you clear the trees and then pull the power back to about 14500rpm to cut out the system. This leaves "wet" power still available if a fan quits.
Each engine has a tank in the aft wing root (a rubber tank which is a b!tch to change)which will hold about 30gal. Thus you only get 5 takeoffs max out of a fill-up (10 gal is min for dispatch). The water burns the hell out of the engines (high TGT's and torque) and costs a fortune, so many "bush" operators don't actually start burning it until well after the book says to.
It's really only used for well loaded takeoffs anyways.
On a side note there was the case of a 748 that got a full load of water/meth just before departing Churchill Man. quite a few years ago. The crew switched the water on to demonstrate the performance boost to a jumpseater or somesuch and blasted off. They got a huge boost and unbelievable performance. Through a couple hundred feet altitude both engines flamed out and they managed to safely land on the (fortunately) frozen river. It later turned out that an old water/meth drum had been filled with a witch's brew of waste oil/thinners/fuel, and the contents of that drum ended up in the tanks.
What a ride! They ended up towing the plane back up onto the airport on its own wheels and I believe it's still flying today.