Dry fasting is what you said no intake at all. The body uses the water in it's fat cells for it's needs and therefore the detox- and symptoms!- can be up to three times as potent. It is considered most humans can safely dry fast for 24-36 hours so that's a good way to start. Extending from there up to 3-5 days which again most humans should be able to go for keeping an eye out for signs of dehydration. Thirst is the main sign you should look for though- but just like our body can trick us with false hunger, it can also trick us with false thirst. The simple test is to wait 30 minutes to an hour after feeling the thirst if you are unsure and if it is still persistent after that to have a few sips every thirty minutes or so until you feel satisfied again. But in learning to dry fast you are learning about your body and it's always best to err on the safe side and break the fast if you feel any concerns at all that it is becoming unsafe.