I mentioned above that I hiked a local mountain on Saturday in 85* weather (daytime highs reached 100.) I live in the desert so the air is very dry. We killed a liter each before reaching the top which is about a one mile hike.
Sunday I hiked the same mountain and incorporated a six mile loop. We started at 6am and the temperatures were 64* with a breeze. We both drank a liter each in the six miles and would have been fine with less.
Just wanted to show what a difference it can make when you change one variable.
As I mentioned before, when I plan a trip I try to determine if I will hit a water source at least once every three miles. If so, then I'll carry 20-32oz of water depending on the conditions. If not then I adjust accordingly.
I still stand by my previous recommendation. If you are really not sure how much water you will need, I'd tank up at the trailhead; drink until your urine is clear. Carry a couple liters on your hike and see how much you have left at the end and use that as a baseline for future trips. Add more when hotter, more elevation gains, less when not, etc.