You can almost have as much as you want...
In fact, there's a diet guru from California (a.k.a. the land of diet gurus) who says you should be drinking between one and two onces per day for each pound of body weight. He claims that if you hydrate "properly" (and by properly he seems to mean "way more than any actual medical expert reccomends"), you'll lose 5 pounds a week even without diet and activity modifications.
I don't know about that, but then again I never did manage to drink all of the water he said I should be (at my high weight, that was almost eight liters a day). Most people say that eight 8-ounce glasses a day is good, which works out to about 2 liters. I drink around six liters a day on days when I work out, and five per day on others. Then again, I have never liked juice, and I can't have any artificial sweetners, so water is the only cold non-alcoholic beverage I drink. It helps keep me full, and often satisfies my hunger. I've also noticed fewer headaches and digestive problems, and, of course, very consistent weight loss.
That said, there is such a thing as too much water, and if it happens, it can be very dangerous. So-called "water intoxication" is all the more dangerous for being rather obscure, and because it--oddly--shares some symptoms with dehydration. The electrolyte balance--and most significantly the sodium balance--in the body becomes diluted by extra fluid. Warning signs include drowsiness, lightheadedness, and weakness of the limbs, followed in extreme cases by delirium. Most often, serious athletes can be at risk, but there are cases of it happening to regular people who have simply mis-judged their fluid needs. Be especially careful not to over-hydrate after a hard workout or time spent in a sauna.
Generally, it's not advisable to drink more than one liter in an hour. The key is to listen to your body and be aware of the signs. You'd begin to feel nauseus before you managed to drink enough to adversely affect your body chemistry; as is often the case, your body would warn you if your intake had become unhealthy.