smiley

How long is the appropriate time to carry a smile, when you pass someone in the hallway?

Example : I pass by Steve and smile and say "good morning" and he returns the greeting.

Once I have passed him, how long do I hold the smile? And then when you release, are you supposed to slowly let it slip off your face or is it ok to end it abruptly? Do the rules change if you are really smiling - like at something funny/happy?