Direct mail...how many times

I have read that the average person does not respond to direct mail until after the 4th mailing. I just got the direct mail software from findmotivatedsellers.com FOr Tax liens, and Banckrupcy leads, he only has it scheduled to send two letters. SHould I be sending at least 4 different mailingds over the course of a few months to every category of leads?
Thanks
Dan