I currently try to fast for 3-4 days but what works for me may not be the right answer for you so I encourage you to listen to your body.

I wrote an article with links to research on different types of fasts, etc., here:

https://medium.com/wholistique/fasting-and-cancer-1eb01904f5de

The article talks about easing into fasting, which I had to do, the benefits of fasting (such as autophagy) and provides links to references.

I hope it helps you!! Feel free to ask questions.