^I think the only way to do that is to specify for how long you want it open when you post it. But sometimes it's just left up to the poster who runs the contest to declare a winner after a couple of days.
The voting will never end. In the long run, I will triumph. It may take months. It may take years. But I shall prevail. Bwahaha.
I would have put an end-time for it, but seeing as how varied it was, I didn't have any idea how long it should have been kept open for. Doesn't matter anyway as the winner was already announced. But it's something I'll keep in mind for the future.