Basically, the shorter the fast the less the benefits, and where conversely, the longer the fast (within reason) the greater the benefits. Even a series of short fasts were found to be not nearly as beneficial as one long fast amounting to the same number of days.
Also consider that healing and cleansing take time which are both dictated by the body, and which in turn depends on your levels of vitality; if some one is enervated/emaciated then the benefits take longer too.