Water quantity is a personal thing, period. All I can tell you have run out taking two liters and 4 liters. It depends on many factors heat, humidity, work load, etc.
The rule of thumb is 16 to 24 oz. per hour activity...I regularly weigh 2 pounds less after a 30 to 40 minute run. Those who advocate a 2 L carry or drink 2 L come off the Trail Camp to Trail round trip dehydrated.
I always take 4 L. I am usually into the 4th liter on the when I retank.
Finally, if you are stuck up there for any reason, don'cha think it would be nice to just tad bit of water?
