Do one complete rep, come halfway up, and lower back down again. That's one rep. Do each rep continuously without pausing and resetting on the floor to keep constant tension on the legs.
I like doing these at the end of the workout as more of a finishing exercise to make my legs really hate me. It won't take much weight at all to have your quads begging for mercy, so start light.
When I get up the balls to do these, I'll do just 1-2 sets of 8-10 (that's 8-10 "1.5" reps) because, well, that's about all I can handle.