I can do a one day fast... and that is it. But I think I did 5 days at 600 calories or less, and one was liquids only.
It was not easy. I think I just prefer the NMN.
I think if you treat it like a supplement you would skip 1 or 2 days per week to avoid homeostasis. But this would generate both NMN, raising NAD+ and promote autophagy of senescent cells. For free!