I suggest that a new open thread be opened whenever the current one reaches 500 comments, since that's the highest number that can be displayed at once, unless a new one would be opened shortly anyway. (Shortly meaning in less than somewhere between a week and three days, depending on how fast these open threads are filling. )

Poll: Vote by voting one option up and the karma balance down.

EDIT: Changing the poll options in light of khafra's comment, "in favor" -> "New thread after 500 comments" (4 votes at the time of the edit); "against" -> "no new threads" (0 votes) and adding options for other thresholds.