Umm, I'm not a fan of treadmil work for much other then gettign back into long runs, so this is for road work.
Urban posted a set that goes
45 off, 30 on for 5 sets
30 off 20 on for 5 sets
20 off 10 on for 5 sets
going from one to the next to the next.
I'd try that if I were you, or do a 2:1 ratio, ie 30s on, 1 min off.
Really though you need to vary it every month/8 training sessions. So one time frame you do the 45 off 30 on thing (which comes to about 13 minutes, add where you see fit I say), one time frame you do the 2:1 ratio (for 20 minutes) and another time frame you do the 1:2 series (ala guerilla cardio, 20s on, 10 off), so you work the different energy mechanisms. And every now and then take 2 weeks off and do 30 min long medium pace runs or 20 min fast pace runs at the same intensity.