Like Zen says, every one is different.
Take a fruit bar or something with you and eat it if you need it.

On steady training rides up to two hours, I don't eat anything. If I have had good meals in the previous 12 hours, thats usually enough. But I always take a bar with me "just in case" on longer rides.

If my ride is only going to be about 60-90 minutes, I usually don't take anything to eat wit me at all.

I race most Saturdays, and the only time I take food with me is on on the longer (30-40km) hilly courses. I don't take food for the flat ones cause I finish those much more quickly, and I find I am more likely to need food in summer than I do in winter.

Have fun finding out what works for you... cause that'll mean lots of riding and that can only be a good thing