The prognosis for individuals with Bell's palsy is generally very good. The extent of nerve damage determines the extent of recovery. Improvement is gradual and recovery times vary. With or without treatment, most individuals begin to get better within 2 weeks after the initial onset of symptoms and most recover completely, returning to normal function within 3 to 6 months. For some, however, the symptoms may last longer. In a few cases, the symptoms may never completely disappear. In rare cases, the disorder may recur, either on the same or the opposite side of the face.
Another complication can occur in case of incomplete or erroneous regeneration of the damaged facial nerve. The nerve can be thought of as a bundle of smaller individual nerve connections that branch out to their proper destinations. During regrowth, nerves are generally able to track the original path to the right destination - but some nerves may sidetrack leading to a condition known as synkinesis . For instance, regrowth of nerves controlling muscles attached to the eye may sidetrack and also regrow connections reaching the muscles of the mouth. In this way, movement of one also affects the other. For example, when the person closes the eye, the corner of the mouth lifts involuntarily.
Roughly, from the studies, it seems that if you do not take steroids you have about a 15 in 20 chance of full recovery of the nerve function. But, if you take a steroid medicine, you have about a 17 in 20 chance of full recovery. So, taking a course of steroids does not guarantee full recovery of the nerve function. However, it increases the chance of full recovery compared to no treatment. You should start the course of steroids as soon as possible after the onset of symptoms; ideally, within 72 hours of symptoms starting. They may not have much effect if they are taken after this.