listsource list updates?

[John Milliken]

wondering how often do you guys/girls repurchase you data list on average?

got my first target area list for my first letters to be sent, and i've read multiple time about sending to a particular list multiple times in a few months span.

do you update the list after 3,4,5 direct mailings or is it a certain amount of months/year(s)?

looking forward to read your opinions.