It depends on the Admin and things such as how busy he is in R/L and so on ..... if it show in your Sent Items (as opposed to your Outbox), then this suggests that it has been seen but they haven't yet had a chance to respond.
Or that the Admin has indeed responded, but it's HIS message that got stuck in the system.
Certainly googling around reveals that other phpBB3 forums have issues with the PM system, but it may have been resolved by now.
Which Admin did you PM? ChrisRLG is also a moderator on the phpBB forum, so if he doesn't check in to this thread it might be worth searching the phpBB forum itself or asking him directly. But try the phpBB forum first:http://www.phpbb.com/community/viewforum.php?f=46
Hope that helps!