3 x 6 min with Level Change
Pre-set the monitor for a work time of 6:00 and a rest time of 3:00. Let your stroke rate vary as follows per interval: 

First and second interval: 2 minutes @20, 1 minute @24, 2 minutes @22, 1 minute @26 
Third interval: 1 minute @20, 2 minutes @24, 1 minute @22, 2 minute @26

The change in rate helped me to stay locked into what I was doing and made the time go quick. Found it easier to stay on track by setting monitor for variable intervals rather than 3 6-minute rounds.