I don’t tend to eat until around 40 miles (on a century ride)
but everybody is different. i start eating from the word go.little and often. on the tour of flanders sportive i got through 3 waffles, 5 energy bars, 2 small gels, 1 big gel and a cheese and ham croissant plus 2 nun tablets, about 3 bottles of water and 1 bottle of dilute carb drink (prefer plain water).
if you bimble along you can get away with less but i prefer to get a move on and was riding with some elites/1’2’s. knocked an hour of my previous years time and was flat out for the last 20k, no chance of me doing that on a flapjack and a packet of maltesers.