Notice the link at the top of the screen called Unanswered Threads? Many contributors click that link first to see which posts have not been answered yet since their last visit. Regardless of how long you wait, once a reply is posted to your thread your thread would never show on that list at all. So bumping your own thread as the first reply is always probably a bad idea.

I would recommend waiting at least 24 hours to bump. Most active contributors come by at least once a day, that gives everyone a shot at your thread on the Unanswered threads list, at least until someone replies.