PilotFighter brings up a really good point. Watch what your student's performance does over the course of the flight. It's typical for a pilot's brain to turn to mush about about the 12min mark, and they start to make a lot of mistakes doing things they did perfectly well for the first bit.

However, it varies wildly. I have one student now who takes the first 3-4min of the flight to settle down, he's so keyed up that he's not really learning anything until at least 5min in, and he usually is best up to nearly 20min.

Anyway, knowing when to break off and land to give the student a break is useful. It helps keep the frustration level down and builds confidence.

On another topic, sometimes you have to hold a student back because they think they are ready to land or do whatever when they aren't. And sometimes you have to push them forward because they clearly have the skills, but are worried and lacking in confidence in their abilities. Controlling the pace and making sure they don't stagnate or get ahead of themselves is important but hard to judge.