How long can someone who voluntary committed them self to a mental hospital be held?

Okay so my boy friend is in a mental hospital. He has bipolar disorder and some times gets angry easily. At like 2 in the morning he got pissed at something and punched a hole in the wall. His mom called the cops saying she was scared of her son and they came and was like you need to go to a physc ward or whatever and he said okay I'm fine but you can take me and do all your tests on me to show I'm okay. So he agreed to go. He's not mentally unstable and he's 17. How long will they hold him. This happened Thursday morning.