Here are a couple recommendations for ensuring that you're properly hydrated prior to the start of a workout or race:
*** 80-100 ounces (roughly 2.4 - 3 liters) of water during the four hours prior to the start of the race; ceasing consumption about 20 minutes prior to allow the stomach to empty.
*** 500 ml (about 17 ounces) of water about two hours before exercise.
*** One liter of water (about 34 ounces) in the two hours prior to the start of the race (about 17 ounces per hour), ceasing consumption about 20 minutes prior to the start.
Each of these recommendations has at least some research backing. Still, you need to determine what works best for your system and the particular logistics of the race or training session ahead. Also, if your workout is in the 2+ hour range or longer, make sure you're drinking water only (no calories for 3 hours prior to the start!). If it's under 2 hours then the use of HEED is perfectly acceptable.