I ring them 15 minutes before my appointment and ask what the waiting time looks like. One of the things I like about my GP is that if she thinks she needs to spend more time with someone, she does. That's great if it's you getting much needed attention but it does mean she is not always on time.
She handles lots of high risk patients, it's almost impossible to get on her list and certain patients automatically get a long consultation booking which I assume gives her a bit of space to catch up.
So, I ring and ask then if she's running behind I go and have a coffee or a wander around or sit in the car and read-the paper. I've trained them to call me just before they need me (that took some doing) which takes the stress out of waiting-- which I suck at.