There was a time when I would undertake a liquids-only fast from midnight on Mardi Gras/Shrove Tuesday until after an evening Ash Wednesday liturgy...twenty one hours, more or less.
Now as Orthodox Christian clergy it is pretty much 12 hours every Saturday night until after Sunday Divine Liturgy--sometimes more depending on how many "extras" follow the Liturgy.
We Orthodox laypeople do the 12 hour pre-Communion fast, too!
No Harvard or Pali connection required!