I can't help you with the grades because I run hills near my house and have no idea what the various grades are. However, after a 20 minute warm-up run, I come back and do 5-10 hill repeats then finish my run. Depending on how many I'm doing they last anywhere between 30 seconds a minute with an effort to keep my cadence at 28-30 foot strikes (i.e., a left foot strike is 1, left foot again is 2 etc.) in 20 seconds. Hill work doesn't need to be a long extended workout, and, unless you have some good running under your belt, should only be done once a week, same as speed work. Now for the disclaimer: I got all this information from my coach. Good luck!