I would like to emphasize -- forget about what is fashionable. Severe dehydration is what's really uncool. Running out of water is a horrible thing on a hot day. And btw today I did a big club ride and there were plenty of people riding road bikes while wearing camelbaks.

I generally aim for 1 bottle per hour, and for me a bottle is 20 oz. I think I aim for that because I've read it in various places. So estimate the number of hours you'll be riding between opportunities to refill (which might be the duration of the whole ride if there's no place to stop and get more to drink) and multiply by 20 oz. Then add more in case you are slower than usual for some reason, delayed by a mechanical issue, etc.

When it's really hot you need enough electrolytes, too. Last year I reread Nancy Clark's book and figured I was not getting enough sodium on hot days. I uppped my intake (using Gu Roctane instead of regular Gu) and I think it has helped me feel better and less "Help me I'm melting" when it's furnace-hot outside.