How much facebook messages can you recieve before the old ones start to go away?

[Neptune]

I am asking, because I messaged a friend on facebook some secrets that I wish I hadn't, that could possibly get me in trouble. I asked her to delete the messages and she refused. I remember being annoyed in the past about wanting to look back through IMs and not being able to see very many, as the old ones were constantly being replaced. However, I've noticed that this is not the case anymore and there is a LOT of messages between us and they are not being replaced. I told her I was lying and so we have been talking. My goal is to continue talking to her until these messages I speak of that she will not delete, go away. Is it no longer possible? Or if it is, approximately how many messages would I need to send her before they start to go away?