Great question, Patrick, and I applaud your key attribute of persistence. My rule is really 5 business days but I say "7" since most candidates I coach seem to miss the "working days" part. So, you are right..5 working days is best. 3 working days is too short, from my hiring manager's perspective. I'm still digging out of e-mails, or I forwarded your application to others and I'm waiting to hear back from them, etc. Here's what I say in my follow up note:  "Ms./Mr., I noted in my calendar to follow up with you on my application (attached) and..."  So, 5 working days!  Thank you so much and best of luck!  How else can I help you?  Dana