How long can you fast for?
A whole month is just about doable.
I think a whole month should doable for most people. Depends a lot on your BMI when you start but your body quickly switches over to using reserves. It's a long time ago but I did at least three weeks whilst at university and could still cycle the 10 km each way, though really strenuous exercise and hard work can become difficult. I think I allowed myself cups of tea and even the odd glass of ftruit juice but it was essentially an extended fast. Given my current weight I'm sure I could manage six weeks!
There's a nice article over at The Econmist about the current fads: apparently Oura rings are currently all the rage. But, I'm increasingly coming to think of some of these people as I do of professional athletes: most definitely not normal.