How long can a psych ward hold you if your there voluntarily?

In the United States, involuntary holds or commitments can typically range from several weeks to about 6-8 months, whereas most voluntary hospitalizations last several days to a few weeks.

For a voluntary admission: a person generally can leave by giving notice, in writing or orally, that he plans to leave against medical advice, as long as he is competent according to a specified standard - i.e., to understand and rationally appreciate the nature and consequences of leaving such facility or program against such advice