perezoso, it is indeed all down to personal choice and trail and error.
agree with Cragc in that if you are doing anything over 30 start eating 50mins-1hr in...even if you think you arnt hungry , feel ok , or dont need it , just eat something small, raisins, mini mars. it just keeps you topped up .i know some people set their Garmin to beep every hour to remind them to eat something , this is a good idea as when the miles go on you become less inclined to concentrate and forget.
as cragc has said, peanut butter sandwiches in bits, small bits of wrap with marmite or jam, raisins work well for me,some sweets that you like, fudge,crunchie bars are good as they dissolve and no chewing required!! cereal bars with nuts and fruit are good and sustain for a while. i always find past 30miles i want salty food rather than sweet. gels are ok, im preferring the GU ones now as they are thicker and less messy. but how many you can tolerate is down to you