Chatterboxes only keep messages for how long?

New to using Chatterboxes and have set up several with a new account opened this fall.
It appears that messages over a month old have disappeared from the boxes. I had no idea they would go poof and assumed they would remain for much longer. Most boxes have only an initial message or two in them so it’s not as if they are too full.
I wanted some basic info to be in a particular Chatterbox for reference. Think pinned to the top of the thread. Is there a way to do that?