(a) The period of commitment to the department shall be a period of unspecified duration which shall commence and terminate as provided in subsection (b) of this section and the court shall not fix the minimum or maximum length of the period.
(b) The period shall commence on the date the order of certification is made provided that facilities are available, and shall terminate upon the first to occur of:
(1) The discharge of the narcotic addict by the department as rehabilitated; or
(2) The expiration of a period of thirty-six (36) months from the date the period commenced.