Acceptable levels

You know when you talk to someone, and they make you smile, how long do you think it is acceptable to keep smiling for? I've just found myself smiling for about a minute after a conversation finished. I wasn't thinking about what was said, it was only after writing a few lines of code that I realised I was still smiling. I had to force my face to return to it's usual miserable expression. I hope I don't always do this, I shall have to monitor my smiling from now on.