Silent Killer Workout... 20 minute interval workout with sandbag and dip station........ Set your interval timer for 20 rounds of two intervals. The first interval is 10 seconds (rest interval) and the second interval is 50 seconds long. Your goal is to push at your max. effort during each 50 second interval. This workout is a circuit that you will have to repeat twice in order to get the 20 minutes in. Don’t forget to write down your reps.