That varies by state. Here in NH we have IEAs (Involuntary Emergency Admission) for being sectioned instead of holds and they keep you in the ER until you have a bed at the state hospital which can be anywhere from a few hours to over a month, then New Hampshire Hospital AKA Concord can keep you as long as they want. You can leave with a CD (conditional discharge) but that pretty much guarantees you’ll go back just before the CD is up.