As you probably know, a great percentage of your body mass is water. So, obviously, you need to drink a lot of water. But, exactly how much is a lot. Well, it's hard to say, but the general recommendation of eight glasses per day is probably a good place to start. However, this recomendation is for the average sedentary person, which you are not if you're exercising.
Exercise causes massive water loss both through sweat and also through metabolic needs.
Even if you are not exercising, it is a good idea to drink water constantly throughout the day. Rather than try to get your eight glasses at the end of the day, you should really strive to increase your water intake. If you drink a glass when you wake up, one with each meal, and one at bedtime, you've got five glasses right there. The ACSM gudielines for hyperhydration suggest three cups of water two hours before exercise. However, this is aimed more at the competitive athlete.