Personally, I don't think there's any rule for judging when an interest thread is a bust. It's possible that interest in an item exists or will grow, even though the interest thread may not be getting many hits.
There are many reasons that interested parties may not post to an interest thread for a long time. People get busy with work, go on holiday, focus on family matters, or for other reasons don't log on for a while. Others may simply miss the thread. There have been many times when friends from the board drew my attention to interest or sales threads that I had completely missed.
I've seen this happen on eBay and other auction sites time and again: an item goes for weeks without a bid, and then suddenly there's a bidding war between several interested parties. There's really no predicting who will see your thread, and when.
So I'd say that you're the only one who can determine when your interest thread is a bust. It's worth bumping as long as you're willing to keep it alive.