If you fuel appropriately and go slow enough, you can basically keep going as long as you can stay awake, or at least as long as your stomach can handle you feeding it carbs. At least that's my experience. But you really need to stay on top of things and it can be quite hard to keep the carb intake high enough.

for sure, bring calories on a long ride!!!!

I prefer nuts and a Lara bar or granola bar, and an apple. Only water for me, thanks. I can drink some electrolytes before and after if needed but I don't like the phlegm feeling they can leave when I am huffing and puffing from riding