After all... 640 votes should be enough for anyone, right? ;)
Seriously, the poll should be closed 1)when someone with the ability to set a poll has a kickin' new idea and the current poll has been up long enough (subjective) 2)the rate of votes per day approaches zero (assumes predictable vote pattern of type y=1/ax). Of course, you can't close a poll unless there is a new topic. If there are no new topics in the poll queue, then the topic default should be some sort of "Future poll topics I'd like to see are:" question, with options being some seed ideas like "Favorite foo", "Most frequent foo", "Perl foo", "General Programming foo", "Life, the Universe, and Foo".