I've read of people doing it from ten minutes up to 30 minutes at a time. I've heard of 10 sec sprints and 30 second "rests" all the way up to 30 sec sprints and 1 minute rest. Currently I've been doing 40 sec sprints at 1 min rest for 20 min but I think I'm gonna try 30 sec sprint/30 sec rest for 20 min. The main reason I was doing 40sec/1min was because I was doing it swimming laps and 40 secs is how long it takes me to go down and back. Just curious on peoples opinions/preferences of the sprint/rest interval they use and the overall length of time they go for and how well that's working for them. Figured it might be interesting. Anyways hope to get some feedback!