I’ve been doing Keto for over a decade now, with one cheat day a year (Christmas market mulled wine with family). I can guarantee you it can easily[0] be sustained if one is so inclined. OTOH, 6 hours without water sounds neither healthy, nor like something I’d ever want to do.
[0]: With the caveat, that your circumstances need to allow it. I’ve been working from home and cook all meals myself.