How often to buy 30-60-90 lists

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I just bought my first set of 30-60-90 day lists from one of the online sites. I will be doing my first set of mailers as soon as I get them. Of course, I will follow the advice from previous posts about following-up with more than one mailer.

IF (I realize it is a BIG IF) I get a decent response and actually get some contracts from the first list, how often should I purchase these lists. The leads were $.30 each. There were only 900 or so in my area. I assume I should mail to the same list for at least a couple months, since the same names would show up if I bought the lists again.

Thanks for your help.