I have been doing 19 or 20 hour fasts literally almost every day for 2 or 3 years. Still kicking pretty good and my dr tells me I’m in great shape every year at my physical. My brother has been doing one meal a day for almost a year and while I’m not privy to much of his health stuff he’s had no obvious side effects other than steady weight loss and looking better.
I’ve come to the conclusion that you could probably do this indefinitely as long as your food intake is quality and provides your necessary macro and microNutrients.
Good luck with your health goals!