I like to hydrate the night before.
Eat some oatmeal, craisins, and a couple of eggs for breakfast.
Drink a cup of coffee, followed by some more water.
Drink about a liter of water on the ride down.
Get out of the truck and pee for about 5 minutes.
Ride sled about 3 miles, stop to pee for 5 minutes.
Ride until I break a decent sweat.
Stop and eat a delicious hot meal (Hot turkey and gravy sandwich, elk steak fajitas) out of the best mod ever, the muff pot.
Drink another liter of water.
Ride hard and head back to truck.
Drink a nice cold Miller Lite and reflect upon the days events.
So to answer your question...about 2-3 liters, or 60-90 ounces.