In the best case if the phone actually signals a de-registration, then UCM should mark it as such almost immediately.  In the case where the phone does not shut down cleanly, UCM will wait for 3 missed keepalive signals - this keepalive interval defaults to 30 seconds, but is controlled by the 'Station Keepalive Interval' service parameter.

One more question. I assume if the phone is out of the access point/forced shut down, it will wait for 3 missed keepalive signals correct? Thanks!

The registration timeout is the 'worst case' scenario, where the device simply stops responding from UCM perspective.  In some scenarios, like roaming, administrative de-registration, end-user poweroff (perhaps), UCM may get a positive signal or TCP connection loss from the network, which allows it to clean up the registration more quickly (even immediately.)