There isn't a setting for it. So, the retention is as long as the database has space.
I've seen a feature request for this, some instance claims it'd make a difference for their storage management. I doubt inbox storage would be a big factor here though.
Long term I imagine it’ll become an issue yeah?
I'll be real with you: I've had a look at the database schema and tables exactly once. It's a bowl of spaghetti in there. We just sort of view it as a single entity and don't really separate it out in any granular view.
What I can tell you is that the bulk of the storage is federated content. When a user subscribes to a community on another instance, the aussie.zone database caches content from it here for them. So, lemmy.world for example uses a lot more storage than the user messages. Or aussie.zone content if I'm being honest. We really aren't that big an instance. We are pretty engaged though. Our user base is small but active.
Surprised the amount of fanmail I receive isn't overloading the server tbh
You should probably be aware that nothing on the fediverse is private. ActivityPub is, by design, absolutely incompatible with privacy. (This may at first seem ironic, considering how many strong privacy advocates there are on Lemmy and Mastodon. But the key thing is that on the fediverse it's entirely consensual and consists only of data you specifically chose to give it.) You should not consider even your DMs to be actually private, because it's possible for clients to intercept and display them even to people who were not the intended recipient.
I've seen some user profiles have a "message securely" button which I think uses an external messaging service, something along the lines of telegram but completely FOSS and possibly decentralised? OP- may be worth looking into that if you were asking this from a privacy angle
Yeah Matrix is a federated (but not ActivityPub) secure messaging service.
Not sure, but I think it should be up to the user.
