Would it be a good idea to require forum members to delete old private messages (or to delete those messages which are several months old) at least to save space? This might be useful if you are on a host plan that limits that amount of space you can use. However, some members might object to having to delete messages, especially if they have lots of private conversations going on with other members.
I don't think my forum has that many private messages, so I don't think it's a problem.
Anyway, one gigabyte is roughly 1,000,000,000 letters. That isn't all that much storage, in exchange for a LOT of private messages. It's probably a problem for FP, but not for me. 🙂
Expanding on what Cosmic says, let's take this example. If each PM has an average of 500 characters, and 1GB is 1,000,000,000 characters (and assuming you're using only using ASCII characters for simplicity), then you can store 2,000,000 messages in 1GB. It's unlikely that you're going to be reaching that number unless you're running a huge forum (in which case you'd probably have unlimited storage 😉)
Yes, especially private messages over 30 days old. The forum can provide a source that private messages over 30 days old will be deleted automatically by the forum site.