I pay my teacher at the end of every lesson. Quite often it overruns, sometimes by up to 20 to 30 minutes for an hour long lesson. The first time it happened, I let my teacher know that time was up, but he was happy to carry on as long as I was not in a rush, which I was not.
I have always paid for an hour only and my teacher has never said anything. I think he is more of an artist rather than a calculating businessman.
What's the norm here? I don't want to feel I'm exploiting him, but on the other hand as the teacher perhaps it is right that he should be given control of the lesson. He might not like if it is me who time the lesson and remind him when an hour has passed.