Question About Direct Mail Frequency

[Ind Kon]

Hello,
I am a new investor wholesaler and I have a question regarding direct mail. From what I have researched, it seems that we need to direct mail 5-6 times (yellow letters, postcards, etc) to a set of leads on a list before we can actually see any results.

My specific question is:

Say I have a list called list A with 500 leads from people from specific zip codes that meet my criteria. What happens after I have mailed those people 5-6 times? Do you just move on to another set of zip codes on another list B to mail and never look back? Or after how long is it appropriate to go back and mail again my list A?

Thanks,
Ind