Q: How long should I wait to hear back from an editor? (What do I do if I don't hear back from an editor?)  

A: If you have plenty of time (that is, if your idea is evergreen or, e.g., pegged to a holiday a month away)  you might give an editor a week or more before you check in.  However, if your idea has a very short shelf life (pegged to breaking news or a news hook that will only be good for a few days) you need to check in fairly quickly — with 48 hours, or perhaps even within 24 hours.  The trick is to be appropriate, not demanding.   You might write a follow up email telling the editor you are checking in on the status of the op-ed you submitted, and hope they are interested in running it; however since the news hook is timely if you don't hear from him/her by the end of the day (week, whatever),  you will assume they have passed and you'll be submitting your op-ed elsewhere.   The key is to be polite and not presumptuous – remember that editors are busy and juggling lots of ideas at once – you are not the center of their universe, but if your idea is timely and good for their readers, they will appreciate you checking in.