• Studies have shown that the natural history of Bell's Palsy without treatment is such that 85% show signs of recovery within 3 weeks of symptom onset, and 71% experience complete recovery.
• Of the remaining individuals who do not completely recover, 13% experience persistent mild sequelae and 16% have residual weakness, synkinesis, and/or contracture.
• Those with incomplete lesions (i.e. incomplete paralysis) are more likely to return to normal function (94%), while only 60% of those with clinically complete lesions return to normal function.
• Herpes zoster is associated with more severe paresis and a worse prognosis.  When little to no recovery is seen within the first 21 days following symptom onset, the prognosis is less favorable.

References

1. Jabor MA, Gianoli G.  Management of Bell's Palsy.  Journal of LA State Med Soc. 1996; 148(7): 279.
2. Peitersen E.  Bell's palsy:  the spontaneous course of 2,500 peripheral facial nerve palsies of different etiologies. Acta Otolaryngol Suppl. 2002.