How long would you wait for your appointment if your GP is running late? What’s a reasonable amount of time before leaving and rescheduling? Especially if you need to be somewhere eg head to work.
Note, this isn’t a one off. My practice runs over every single time, over the years the standard wait is 20-30 minutes but has been up to an hour - once there was even a 2 hour delay! (I had to wait as it was important).