Could a person with avoidant personality disorder need therapy lifelong?

Depends on person: Avoidant personality is responsive to therapy but the length of therapy depends on your state of disorder and the techniques used and also expertise of the therapist. Your willingness to be assertive and try the taught skills also counts.

No: It's very difficult to treat or cure a personality disorder. Instead, a doctor will work with the patient on treating the symptoms that make up the personality disorder. In this case the therapist may work on possible accompanying depression and anxiety with the patient.