As little as possible. Your goal is to get the maximum number of sets in 20 min., so this is a real cardio workout and you'll be huffing and puffing by the end. A beginners tip: if you have a problem with burning out too soon because you take no rest during the first bunch of sets and then take too long a rest near the end, try and schedule a set every 1 min. or every 45 seconds. This will help you pace yourself until you can figure out your own rhthym.