for an extended period of time
Wait so you posted the other day asking if 400-500 calories per day was enough and now you’d like to lower the bar to 0 calories per day?
To answer your question even though it seems obvious, you die. If you only drink water you might survive around a month at best depending on your current body fat levels. You’ll be skin and bones before you die though.
It’s called fasting. Many people do it for extended periods of time. Before doing that, you need to study up on it to do it safely. Visit r/fasting.
That said, I would not do it right now because prolonged fasting wipes out your immune cells temporarily. Not a good thing during a pandemic!!