Probably much more often than you think. Keep in mind that most direct purchases only occur when an order form lands in front of a prospect. The more often a prospect sees an order form, the more often he/she is likely to purchase.
Re-sending exactly the same direct marketing package and offer to the very same list within 90 days or less will often produce 75% to 90% of the original response, and not infrequently the same or more.
Discover the optimum frequency for your offer by doubling your frequency and measuring the response. If it is still profitable, double it again. Keep doubling the frequency until response falls so low as to be unprofitable. Then step back one level and continue marketing until response becomes unprofitable again. Settle on the best frequency for generating the largest annual response possible from each list.