Someone mentioned a colonoscopy and so I have had several days were I am just drinking regular mountain dew, broth, and sherbet but that is still eating and getting calories. I don't think I have ever went very long involuntarily not eating. I used to do a fast where I would only non calorie liquids so water, diet soda, tea with no sugar, etc on tuesdays and thursdays. So figure from something like monday 8pm to wednesday 8am. So something like 36 hours but 16 of those were sleeping so not as bad as it sounds. This was in my forties I think and I likely may have been able to do it in my thirties but I can say I could not go that long when I was younger as my metabism was fast and I would get a headache or such.