I've been using this set of exercises as a less-intense interval routine when I don't have time to do a full p90x workout, or don't feel like making up my own set of intervals for a day. Two times doesn't feel like enough for me, but I was pretty tired after getting through four once. Three will probably do it for most people, I'm small so I can do an unusual number of reps for bodyweight exercises. I like it since it's fairly focused on the core and hits the major muscle groups. You can always make it harder by changing the normal squats to jump squats, doing a different ab move, etc.