If you are voluntary they can't keep you. They can make recommendations and suggest you stay but they can't stop you from leaving. Now, one can voluntarily admit themselves after which they can apply to a judge to have you committed. In my health region that is generally 7-10 days.

When I have been hospitalized it was voluntary. I couldn't simply walk out though. I had to secure a pass to do so. It was necessary for me to communicate where I was going and when I expected to be back.