As often as you are able to deliver new insights that your prospect finds interesting, you can contact him/her. Contacting a prospect is never about sending the stupid note that says, “did you see my list email?” or worse yet, “I’m just checking in to see if you’re ready to talk.” Excuse me, but “checking in” is what you do at a hotel, not with a prospect!
What you can’t afford to do in prospecting is fall into the trap of believing if you try to contact them more than a couple of times, you’re bugging them. Or, since they haven’t responded yet, they must be disinterested. Both of these are beliefs you can’t allow yourself to accept. The only belief you can have is one that believes in your ability to help them see and achieve what they didn’t think was possible, and therefore, it’s your responsibility to reach out