Generally you want to plan for at least an hour for most portrait sessions. It might take a bit longer if we're doing something really unique or complex...but the good news is I charge a flat rate for the session and portrait so if things do end up taking a little longer than planned it doesn't cost a penny more.
Some times it does take a little longer to get that perfect shot. Other times, boom! Something good happens in the first thirty seconds. C'est la vie.