This session can be “hacked” to be 20min, or “filled out” to be around 30min. In any case, do not go longer than 30min, because your stress hormone release will sabotage the effectiveness of the session. More is not better in this case!
Justification and further resources for the 8sec “Sprintervals” can be found at http://www.lifehacker.com.au/2013/03/researchers-says-this-is-the-single-most-efficient-exercise-regime/ .
Sprintervals are 8 sec sprints, followed by 12 sec rest (“active” rest, which means you still move, but slowly). They are performed for a maximum of 20 minutes.
Today we did 2 x 5min blocks, plus some core training in the middle. To flesh it out, do 2 x 10min blocks, or the full 20 min with the core training afterwards.
First, perform your chosen session time on the cardio equipment of your choice. The bike or rower are good choices because it is difficult to hurt yourself if you get tired (like when running).