Deciding the time frame that you use to retarget both prospects and purchasers from your website make a big difference. Visitors that leave the site without buying have the most potential for coming back and purchasing in the first 14 days (of course the tricky part is figuring our whether they would have come back anyway without seeing your ads). The more time passes, the colder the prospect gets in terms of re-activating them as a customer. Past purchasers are just the opposite. Right after purchase isn’t the smartest time to serve them ads since they just purchased, but depending on your product, the more time passes the more valuable they are to message.