Not really interested in some extreme ride you do once in a blue moon but rather your regular average.
For me it is about 4 hours. At this point I hit a wall without food.
I haven't yet tried going past this with food. Before I didn't have a bike bag and got used to not riding with a backpack but have bought a nice 18L one which will give plenty of storage for food if I so wish.
Generally I don't feel like I want to go longer as at about 2 hours, considering the round trip, I think to myself that there are other things I could be doing with my day besides riding and 4 hours is a good amount for me to freshen the mind.
On the other hand there are some very interesting landscapes nearby, one of which is a national park I have yet to explore in any depth, that lay just out of reach while restricting myself to only 4 hours so have thought about pushing on. When I am on the usual timed ride I will see them just on the horizon or even reach within the perimeter, but then the tank was empty since no food and was just wanting to get home.
How much food do you will need to do another couple of hours I.e. up to six. 18 litre rucksack will probably be good for 3 or 4 days food shopping. Just stick a sandwich and a couple of grain bars in your jersey pocket.
Or when you need to top up your bottles buy a sandwich or go mad and stop at a cafe.
