You never know what story will get picked up. So you have to keep the pipeline of submissions full at all times. This does NOT mean that you submit one story and sit on your thumb until you get an answer. Well, you could, but you’re not going to progress quickly that way.
What you want to do is have a constantly orbitting ring of stories that you submit, get rejected, request feedback, edit, and resubmit until they sell. You decide the size of the pipeline: 5 works out there, ten, twenty, it’s all about your scheduling and work capacity.
Right now, I have seven out there, by 2/1/12, there will be ten works out there (I hope!)