Hopefully you know that dehydration increases the risk of DCS. I realize it's not exactly what you're asking, but as Charlie99 said, if you're feeling very, very thirsty, you're already quite dehydrated. This is not actually all that unusual; many people go around every day slightly dehydrated.
To put numbers on it, by the time you start feeling thirsty (not necessarily just dry mouth), you're short on water by at least 1.5-2% of your body weight. In other words, if you weight 150 lbs, you're dehydrated by at least 2-3 pints (4-6 cups). If you were to drink this much pure water, it would take 60-90 minutes to be fully absorbed into the blood stream, and even longer to diffuse into all your other tissues. And if your stomach has a lot of food in it or you're drinking stuff with a lot of salt or sugar, the absorption rate goes down. However, somewhat surprisingly, absorption rate isn't as affected by body size, so a big person is going to need more water (or water mixed with other things) and is going to need more time to absorb it adequately.
Bottom line: drink enough to make up your shortages a few hours before diving, and keep drinking until you enter the water. One medical type even posted on SB once before something to the effect that 'if you need to pee at the start of the dive, you're adequately hydrated.'
