How often you treat is really up to you - personally I never treat more than once a week for any condition unless it is incredibly acute (falls, accidents, swelling, bleeding, etc.). Many others treat 2-3 times/week on a regular basis for all types of cases. Unless it just started recently I've never seen any better results treating multiple times/week vs. once a week - but that is just my experience.
Bell's Palsy is well treated with acupuncture. Cupping is appropriate on the upper back to 'expel wind' and moxibustion is used by some practitioners on the affected part of the face to increase circulation.