How Do You Approach Testing and Application?

There's this dynamic where you can test unlimited things and use all of your time to make sure that the sum function of a given language is working properly. Where do you draw the line between over testing and yet being confident in your code? Anything that I write from scratch, I try to be fairly exhaustive. My functions, I, there's an interesting book from Manning called Five Lines of Code,. The general idea is that a function should be about five lines of code.