That's a short timeline and a perverse incentive. One of the clever parts of SO bounties is that they're taken from you whether you award them or not, so the incentive is to be a nice person and do so. Here the incentive is to never award and hope you get it back.

I understand you don't want people potentially throwing away money - its a problem where you need a high volume of answerers for the no-refund model to work. Just something to think about.