My instinct would be to wait NO LONGER than two weeks. If my submission is still sitting in their queue after that duration, with no peep whatsoever from anyone who runs the Publication, then I think I can safely assume they're probably never going to publish it -- and it's time to withdraw it from their queue and re-submit it to a different Publication (or go the self-publishing route, but I'm hesitant to do that most of the time).