How detailed does a paper-mockup have to be?

The question looks silly but I'm currently working on a small project that involves creating a paper mockup.

The point is to learn and understand how paper mockups help to do user centered design (UCD). My problem is that I'm not sure whether I should model every "if-then-else" aspect of the system, because doing so would result in a very large mockup.

Are there any people out there who have some experience with paper prototyping?