I've been doing some experiments with my wife over the last couple of years. We tried 1 day per week, 2 days per week, extended fasting of various sorts (up to 10 days). We both found that there is a significant shift in mood and mental clarity on day 4 (3 if you're lucky), when you enter really deep ketosis (might also be related to increased autophagy), so now we settled for 5 day fasts every fourth week and it works like a charm. I guess you should start slowly, but don't be afraid of experimenting with longer fasts as you get more comfortable, just keep in mind to give your body enough time to regenerate.