I used to drink ~6L of Mt Dew in a single day. I did for several years (all through high school, and for a while before that). The one day it clicked that I was consuming as many calories in soft drinks as I was in regular food.
At the time, I was highly active (hiking, camping, backpacking, skating, ROTC, etc), so I didn't think much about it. Once I got to college, though, it was like I had been chained to my desk.
Switched to diet drinks (partially to help my grandfather, with whom I was living, with he new heart-conscious diet) and dropped 30 lbs over about six weeks. I didn't change anything else, I just cut out all the friggin' sugar from 3L of Mt Dew per day.
These days, I'll drink a tall class (32 oz) of water when I wake up, a 20oz diet (either Diet Coke or Diet Mt Dew) before lunch, a cup of whatever w/ lunch (water, diet drink, sweet tea), and a 20oz after lunch.
Then I'll usually drink water until dinner (another 20oz if I need it), soft drink or tea w/ dinner (water doesn't have enough flavor 🙁) and drink water until I go to bed.
I try not to drink any caffeine after dinner (I have chronic insomnia, and the caffeine hardly helps), but I have to have the caffeine during the day just to survive. I may try to wane myself over Christmas, but I can't afford that kind of crash during the semester.
After I graduate (next Christmas!), I plan to switch almost entirely to water (and tea or diet drinks for my meals). I'd love to break my caffeine addiction but, like I said, I can't afford to crash in the middle of a semester.
All said, I probably drink 60-100 oz of diet soda and 8-12L of water in a given day. Much better than it used to be, that's for sure. 🙂