How Many Times to Hit a List

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I have a few different lists that I market to and send a letter to every 8 weeks. I usually alternate between a handwritten yellow letter and a professional letter. My question is how many times do you mail to a specific list before you stop or do you equate keep mailing forever. I'm currently on my 8th mailer I think to my Absentee Owner list for example. Just curious on BP members thoughts. Also how many times do you hit a list before giving up on it if you have gotten 0 deals. I wholesale BTW. Thanks