Default to never, then the lower timeframe chosen by one the two parties will be mandatory.

So if A sets 90 days and B sets 1 day. Then 1 day is going to be the message disappearing time of the chat.

Also a user can decide to have all of their chats default to 1 day or 1 week and all of their chats will default to that.

If A and B initiate a new chat and A set 90 days as their default and B set 1 day. The chat will have a 1 day default timer.

In this way if 2 users do not care about messages disappearing they will have persidtency, if any of them cares they will have a right to privacy and the greatest privacy expectation will be always respected.