I'm currently practicing ADF (alternate day fasting) although I fast on M-W-F rather than true alternate days. I go about 42 hours of black coffee/water and by the time I eat a meal it's about 48 hours between solid foods.
I'm finding that's long enough to "feel" changes happening within the body. My body feels like it's working much better than before--much calmer. My skin and teeth are showing improvements, etc. I saw a table yesterday that said 24 hours won't produce a high level of ketones (I assume your true level will depend on your context and physiology) but by 48 hours you will have a high level of ketones.
I'm 2 weeks in and it's definitely been worthwhile but I'm not sure how long I'll continue with 3 days per week. I'm considering either shifting to 2 days per week or staying with 3 days but fasting only every other week.
EDIT: Ned Knock of Health Correlator provided a link to the table in a comment he made on Free the Animal.