When to leave shift ?

Another CNA said today you can't ever leave a shift, when your shift is over, until the nurse clears you to leave. Like if your shift ends at 10 pm, you stay until 10:15, but the nightshift still has not shown up; and you really have to leave because you have another job to go to, or family responsibilities at home. I am not sure how they could keep "holding" you there for a long time after your shift is over.