A.C.
In response to another answer: actually with an anti-lock braking system you AREN'T supposed to pump them (at least this is what I've been told), the ABS does the pumping for you.
Slamming on the brakes shouldn't happen often, but EVERYONE has had to do it at least once. Knowing they will work properly is a good thing to know.
Speed, weight and road conditions all determine how much room you need to stop. Larger, heavier vehicles need more room, as well as faster speeds.