A voluntary patient who is 18 years of age or older and who desires to leave a treatment facility shall submit to the facility a request to leave on a form provided by the facility. When the investigation is completed, the patient shall be evaluated immediately in writing and discharged immediately or given written notice that involuntary commitment proceedings will be initiated against the patient. The treatment facility may detain the patient for no more than 48 hours after receipt of the patient's request to leave in order to initiate involuntary commitment proceedings.