The department shall keep every person committed to it under s. 975.06 under its control and shall retain the person, subject to the limitations of s. 975.12 under supervision and control, so long as in its judgment such control is necessary for the protection of the public. The department shall discharge any such person as soon as in its opinion there is reasonable probability that the person can be given full liberty without danger to the public, but no person convicted of a felony shall, without the written approval of the committing court, be discharged prior to 2 years after the date of the person’s commitment.