I have been discovering similar experiences. Seems like I do best at about 16 hours, unless I am going for a 24. That is LeanGains protocol, right? But seems like lately, 10 or 11am is when I start to get hungry, if my day starts at 4am. I try to hold off until 12 or 1, more of a Fast 5.
I dont do near the activity as you though, not really. I might "do stuff" for hours, but I dont know how effective.
But today, I had coffee w/cream first thing, then when out and caught my horse and got ready for a ride. Ride was 4 hours. Probably an hour+ of catching, tacking, feeding, haul saddles back, etc. Anyways, usually when I go for a trail ride, I pack like I might be gone for a week! Today, I took a couple bottles of water and that was it! So proud of myself! So it was after 2, maybe even 3 before I ate a little something.
But in looking back over the summer, I had many 24 hour days. Mostly Fast 5 days, and my weight loss crawled. I never binged. Calories always in check. The 24 hour days were more like 1000 calorie days. So I dont know. I need more n=1 but I do think that I still need to incorporate some 24s now and then, and a 48 every week or two for the health benefits. I keep thinking I should do a longer one, a modified one with kraut and broth.