I’ve asked this same question, had to make this same decision and learned the hard way. But I think it’s fair to say that give it a day, max, if the story is timely. Otherwise, wait 30 days to six weeks for a reply to your pitch. If you don’t hear back, either re-pitch (most magazines welcome this) or move on with your story. Some one else might want to listen to what you have to share.
(thank you, Martin for your question this a.m. – hope this helps).