That's the song they sing...
but frankly, I believe that like anything else, it varies from person-to-person. You have to kinda watch what you expel (be it perspiration or da whizz). You definitely can't go on thirst. 9 out of 10 times, you are reacting a little late. I can drink about 8 oz. in the a.m., run a 10K in moderate-to-cool weather and drink another 16-20 oz. and be good to go! You just have to pay close attention to your activities...watch out for da alcohol...you'll be de-hydrated quickly!
NUFF!