I’ve got 2 PMs that have been sat in my outbox for a day or so….
It might mean your message hasn’t been read by the recipient? IIRC, PM’s stay in your Outbox until the recipient has read the PM. When your PM has is read by the recipient, it’s then moved to your Sent folder.
Out of curiosity, I had a look at a phpBB userguide:
Sent messages will appear in either the Outbox or the Sent messages folder. As long as the recipient(s) have not yet read the message, it will stay in the Outbox. As soon as someone reads the message it will be archived to the Sent messages folder. If the administrator allows it, you can edit messages after sending them as long as they are in the Outbox and the recipients have not yet read them.