14-17 hours is a reasonable fast. [well, it’s reasonable for a wet fast]. Dry fasting though could be rough on the kidneys.
I was Eastern Orthodox Christian for 6 years from age 24-30 – still technically am, although I’m still technically a part of a lot of groups officially [never liked the politics many of them get in but I liked the deeper monk-ish stuff like Theosis and such] and I didn’t mind the fasting regimen. There were lots of fasting periods throughout the year.