Alot of it depends on a metric called frequency. Frequency refers to the number of times each person in your audience saw your ad. If you have a high frequency say 5 or more, and people still aren’t taking the action you want them too, that tells us that it is time to stop that ad or make some changes to the campaign. If someone has seen your advert 5 times and still not purchased something isn’t right.
Annoyingly, there is no ideal frequency because it’s all dependent on time frame. If someone sees your ad 5 times in a month that’s not bad, but if they see it 5 times in a week they are probably getting annoyed. Also if they’re seeing it a number of times in a week then that means your audience is probably too small, or your budget is too high