I did a dry fast this weekend (started midnight friday)
On Sunday night I didn't feel like sleeping much and my body felt stressed but I didn't feel thirsty.
So I got to sleep eventually and then at 5am I woke up dreaming of drinking a cup of herbal tea and my mouth was dry and so I knew I had to drink something so I did.
I've just re-read some of Fontys posts, so yeah basically listen for a dry mouth and thirst and then drink.
To get the most of out fasting try to go for as long as possible without water but always drink when you have a dry mouth and feel thirsty, don't aim to go for a set period of time without water.
Seems like you can train your body to last longer without water over time.