If it's reasonable temperature, I drink about a bottle (20 oz) per hour. If it's hot (upper 80s, 90+) I can drink more than 2 per hour and still be thirsty after a 4 hour ride. If I'm not racing, easy, just stop and refill. I get my calories and electrolytes from my drink mix - 3 bottles of infinit nutrition mix is good for 4 hours, or more if I'm properly loaded on carbs. For road races, which can be 3-4 hours I just hydrate a lot, then bring 3 bottles. If there aren't water handups and it's hot I'll be pretty dehydrated afterwards, but not so much that I lose power during the race.