That's a difficult question to answer as the slope of the ramp makes a big difference. For a true level float off, Red Raven's description/estimate is probably pretty good. However, the steeper the ramp, the less water needed at the wheels. When the boat is at an angle the stern will rise while the front of the boat is still on the trailer. But as the rear of the boat starts to float, carpet gets wet, etc, the friction between hull and bunks goes down dramatically. I find that once the boat is about 2/3 afloat, it slips off fairly easily without and drama of engine revving, wheels spinning etc. I simply pull forward a couple of feet and the boat will follow along but will slip a few inches. Let the truck roll back to the original start point and repeat. Usually only two or three evolutions is needed. This all on a well designed, purpose built ramp. All that said, when doing this I'd bet that the wheels are pretty darn close to Raven's guestimated 40 inches of water. My stopping point is when the wiring harness plug-in is within a couple of inches of getting wet.
For the same reason when hauling out, the 2/3 floatation supports enough weight/reduces enough friction to pull the boat the rest of the way (maybe 18-24 in.) forward.
