Everyone will give you a different answer as everyones body is a bit different and there are too many variables to consider. Regardless of that, you should never wait to eat until you are hungry. At that point it is too late and you may "bonk". Replacing the lost fluids through sweat is important, especially the salt in the warmer months. I always look for a decent amount of potassium in my drinks and usually carry a banana with me if I ride longer than 2 or so hours. This works for me. I'm able to eat up until about an hour before a ride with no problems.
good post, I have an annoyingly fast metabolism (as most 19yr olds do) and have worked out need at least one sandwich/muesli bar/ gel in the first hour and 2-3 each consecutive hour.
Also, I recently did a lap test on dehydration and was really surprised that in an hour of working at 75% of FTP, I lost 923ml of fluid averaged over five tests in a controlled 18 C environment - i.e. thats a fair bit more than one bottle / hr which most people think of as a benchmark.