Not enough. I'm so bad about drinking water, or anything really. I find it a nuissance so I wait until I'm thirsty, which I guess means you're already dehydrated. The only thing I found that helps me drink is lemon in the water, or miracle clay water.
But I do agree that we need more water then 1000 years ago.
I carry water with me all day that I have frozen the night before as I know I'll drink more if it's very cold. We don't really buy any other beverages for the home except some juice now and again. We switched to only water many years ago and it has made all the difference in our health.
I am very fortunate to have some of the best drinking water I've ever tasted here and it actually is preferable to any other kind of sweet drink.
If I had to estimate how much I drink in a 24 hour period, it would be between 3/4 gal. and a gal., some days more.
I'm on a medication that requires me to drink a lot of water. So, I drink a ton. Four glasses of water a day seems really low to me. Even before I was on this medication I drank a lot of water. I just never felt healthy unless I was drinking something every few hours. Usually water, sometimes herbal tea. I guess it's rather personal though, everyone has their own needs for hydration.
Never enough.
But, I strive to make sure the water is not cold. Cold liquids slow digestion drastically.
I do drink lots of raw milk and Kombucha as well.
For those that drink coffee, note that it is a diaretic (sp?) so for every cup of coffee you drink, you need to drink an extra 2 glasses of water.