It depends on the height of your pickup off the tank floor and the angle of the tank and that varies based on trim and weight distribution on the boat. If you move all the way aft, you will have more fuel to burn before you run out but it's hard that way to steer
If you really want to find out, it is doable in an inexact way with two tanks by running on one tank until you run out of gas. Switch to the other tank and then go fill up the first tank. When the tank is full that is the amount of gas that you can suck up. Repeat for the other tank.
Really though, you should never plan on using more than 90% of a tank and even that is pushing it.