Some people have a large reservoir, some small. Some people are more suspicious by nature, or more ornery; others are inherently more patient, trusting, or optimistic. The point is, you can’t count on a very large reserve. It’s idiosyncratic.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.