That's an interesting question...they way I have typically done hill repeats to to pick a specific climb, ride up it, turn around and go to the bottom and ride back up it. Do this until you can't back up (unless you need to be at the top at the end) or until you think you could get back up again. Build the number of times you can repeat the climb. When I have done them I had a 1 mile climb I liked to use...I would do one at high cadence, then one standing the whole climb, then high cadence again and so on.