I would definitely leave it at a week; with anestimated active 5000 monks around, it can easily become a pain to respond to daily requests. I would also limit the max reputation that you can challenge; anything less than 20, for example (and even that might be too high); I can see an abusive monk that has posted well using it to annoy 50+ monks at a time. I would also limit the date of which nodes can be challenged; nodes older than 2 weeks for example should not be allowed, only because the context will be lost.
in reply to Challenging votes
But I do agree this is a good idea; it does remove the ease of those votebots to abuse the system.
Dr. Michael K. Neylon - email@example.com
"You've left the lens cap of your mind on again, Pinky" - The Brain