How Do You Determine the Right Level of Suffering?

In "The Coddling of the American Mind" Greg Lukianoff and Jonathan Haidt discuss the new culture of safety that has developed on campuses around the country, and argue that children and students need challenges and stress and even suffering in their life to develop properly. If we grant their premise, how do we decide how much suffering to introduce? And how do we convince people to accept more suffering into their life? How do we determine the right level of suffering?