I seem to be like a camel... drinking way less than most people hiking with me. Of course using a CamelBak that should make some sense, huh?
But seriously, I think these factors help me go farther:
1. I stay fully hydrated every day whether hiking or not
2. I eat more the night before and very lightly in the morning... (digesting a heavy breakfast uses up a lot of fluid)
3. I stay away from alcohol/coffee/diuretics the night before and the day of a hike (my natural internal alarm clock beats coffee)
4. I drink fluid the normal amount of fluids (see #1 above) before leaving home, then drink maybe 6-8 oz just before hitting the trail
5. Simple sugars or not Gatorade is my staple, thinned ~10% with ice cubes in the CamelBak in hot weather, tap water in cool.
6. I always have plenty of fluids and food in the car
X. I have numerous caches in strategic locations so if I end up in dire straights I have an emergency supply.
(So far I haven't needed any and so far nothing/nobody has touched them... yes, I check often enough)
With that in mind, no matter how much I take I seem to return with plenty of cushion left over. As often as not, I end up with more than half
left so I've been gradually cutting back on what I begin with. Even then I don't find myself needing to drink excessively upon returning from a hike.
By consistently following the steps above, it's been over 6 years since I ran out more than a mile from the car. Although a warm day in the Supes topping out at 96° I logged 20 miles in 9 hours on 190 oz of Gatorade, running out 2 miles from Peralta TH on the Dutchman. I wasn't too concerned as that stretch was easy enough and I had plenty to drink in a cooler in the car.
In the end, it's use your own experience, beginners erring on the high side until you find your happy medium.