As many as you can do in about 20 min..
or
Change your speed/incline every minute. that way you can never get bored to much. 1 workout is pick a comfortable speed to run, say 6.0...this might change for you, I am just saying..
Every minute change the incline up 1 notch every min..so by 5 min, your on 5 incline ..speed is the same here, work up as far as you can go...lets say its 15 incline...Now go back down, lower the incline every minute. while increases speed..hmm, thats the much tougher part..

))
remember you can change this to how ever you want..example, but that workout is 30 min and its a killer!!..