If you mean HN then you can probably guess some time after your submissions and comments stop getting any upvotes and replies, that can also happen just because

If you mean HN then you can probably guess some time after your submissions and comments stop getting any upvotes and replies, that can also happen just because you're not contributing in a way people appreciate or you're not being noticed like comments on new submissions that don't reach the front page. If you mean YC I don't think they do, there's plenty of valid reasons for them to reject applications again and again.