I can tell you my routine. My street is 3/4 mile long and fairly hilly. I start cold and do a light jog to the end of the street. Then I sprint back, stopping to walk in between. I don't time them, but I try to sprint until I feel like I'm about to hurl. Most of the time I complete 5 sprints. Sometimes I turn and go back up the street for one or two more. It usually takes around 17-20 minutes, and then I'm done! You can also "sprint" up a weight workout. Google "tabatas" and have some fun.