Q. How often can I submit to the same person?
A: As often as you like—provided you have a good, timely idea that would appeal to that editor/outlet. You don't want to pepper an editor with bad ideas, and thereby earn a reputation as someone who is not useful/generative. However, if you have had a successful experience with an editor, the best strategy may be to continue your conversation with that editor, try to pitch as many good ideas as possible, learn as much as possible about what s/he is looking for in an op-ed, and see if you can collaborate on additional pieces.