This is a difficult question to answer because it depends on a number of factors, including the type of product or service you're selling, your target market, and your overall marketing strategy.
That said, a good rule of thumb is to retarget your ads at least three times to potential customers. This will help ensure that your message is seen by as many people as possible, and it will also give you a chance to fine-tune your message and target your ads more accurately.
Of course, you may need to retarget more or less often than this, depending on your specific situation. But if you're not sure how often to retarget, three times is a good starting point.