I, too, have tried to drink more than 5 liters and I simply can't do it. I just start to get ill at the thought of drinking more water after that.
I do martial arts 2-3 days a week and weight-training 3-4 days (not so much recently due to a hand injury) and even on my hardest training days when the weather was hot, I couldn't stomach more 5 than liters, and one of those was drunk during/immediately after a training session (not counting the h2o in my PWO; I have had several times where I've refilled after that and drunk another 0.5 liter immediately).
I know if I don't get at least 3 liters every day I will feel like crap after a couple days and my performance in just about everything suffers, especially fighting.
I live at ~6000 feet above sea-level and I've heard that active people up here should be getting between 1 and 2 gallons per day, and I certainly agree with the minimum, but the closer I get to 2 gallons, the worse I feel.
It seems to take about 2 days of drinking less than 3 liters before I start really suffering, and it also seems to take about the same amount of time for me to get back to a good level of hydration.
You just need to always judge by how you feel. I've always gone by the "if you wait until you're thirsty, you're late" and "if you're not always sipping on some cool, refreshing water, you're not getting enough."
However, I can say that on the occasions I get stuck somewhere and I have an empty bottle, I get totally ticked off. For example, after work yesterday I took my wife shopping and I forgot to fill the bottle, and I pull up into the parking lot of the place and reached for the bottle because I was feeling a little thirsty. . . there was maybe one mouthful. I got angry.
So, I have decided that I will always have two one-liter bottles with me in the vehicle, just in case.