Anything >3L without sufficient exercise or activity can be dangerous because you're going to flush out your water soluble vitamins and minerals from what I've heard. This can be potentially dangerous as your body functions can deteriorate! So be v. Careful
Well, the biggest problem is that the most people are not drinking enough, and about the top limits, I don't really know but heard alot of different stories.
I do an hour of cardio everyday at least, and gym 3 times a week or less on busy weeks. I usually have 3L - 4L of water a day. I think with sufficient exercise / gym, having around 4L of water is good, as water helps a lot when it comes to gym => http://www.bodybuilding.com/fun/animalpak21.htm
7L seems overkill... try 4-5L at most, if you are a person who exercises daily. For those of you who rarely exercise, or have a day with no exercise, 2L-3L is sufficient.