Well, because it is a bodyweight tool, it's hard to get injured from it. If it gets to much, your body will just drop to the floor. But it is an entirely different exercise to everything else.
With bodyweight exercises like the ab wheel, pushups, crunches, etc. I don't have a set amount of reps. I do the same exercise until I can't do anymore. Then rest for two mins and repeat this. I do this 3 times.
I ONLY do this with bodyweight exercises though, cause they are alot safer.

If you want to get your stomach ripped, do heaps of crunches and reverse crunches. (Need know what reverse ones are, let me know.)
If you have a little fat covering the ab region, just sweat it off with some sparring or aerobic work.