First Water Fast

I want to go on a Water Fast beginning tomorrow. I am not sure how long I will stay on it. I have previously done a juice fast for 21 days. I have a few scars that I want to get rid of and I want to do some internal cleaning. Can anyone give me any insight on how long I should go if I want to lift these scars. The scars aren't bad/deep they are just annoying. :)

Also, please give me an insight on the whole process.

Thank you!