I probably don't eat/drink enough but my general rule of thumb (for me) is on a long, long ride:

gel and two bottles (1 water, 1 with sugary stuff - Enervit or Gatorade seems to be my favorites) in the first 2-3 hours, then 2 bottles and a gel in the next 2 hours, then a gel or crack pack every hour after that, bottles as necessary (1 per hour seems the rule). This is in 60-75 degree weather, dry, mid level effort average with a couple hours of steady threshold effort (i.e. climbing up hard climbs).

If I feel hungry (i.e. stomach feels empty) then I'll have solids - bar, fig newtons, peanuts, whatever I feel like. If I crave it I buy it.

I rode 2.5 hours the other day - 40 deg, a bit windy, overcast. Brought 1 bottle of water. When I rolled the bike into the basement I hadn't touched the water.

cdr