I started my fast drinking a LOT of water, while my body still had lots of food in it (8-12 cups a day for the first 2 days). Since then I've switched to drinking a glass or two more than I'm thirsty for -- though to call it 'thirst' is a misnomer, someone mentioned noting a dry throat or tongue and using that as a guide, it's worked well for me so far. 5-6 glasses of water sounds safe and reasonable, probably about right (if that's what 91 ounces is).
Also, urine colour is a good indicator -- if it's fairly dark yellow, you need to up your intake. If it's not yellow at all (even in the bowl), you're probably drinking too much.