There are so many factors that can impact the length of the fast and how often you fast.
If you have done all kinds of cleansing procedures and are in fairly healthy state, then you can even fast once a month for 3 days. Some do 1 day a week.
Length of the time and how often cannot never be nailed down because we are all different. Also every fast is different. Some ailments may take a very long fast to clear it. If we talk about just maintenance fast, then again we are all different with different circumstances.
My rule is listen to your body and fast when your body tells you to fast. Also same goes for how often you fast.