I used to do a version of Fasting where I would stop eating around 6 pm on Thursday night and then not eat til breakfast on Saturday morning, so I think it was just over 36 hours.
Once a week, fabulous results. I was a tub of lard though, so I don't know if the results are typical.
On another note, I already lost all the weight I gained this weekend. I must have been out of keto and retaining water because it's already all gone. 6L of water a day will flush you out.