when online.php actually worked, it was 2-3 days. However, someone's been messing with the code enough to break the page that lists online users.
from a quick look I see some users that show up as being online, but have not been on the forum in 2 days.
in comparison, most other sites use either 15 minutes, 1 hr. The laser projector forum uses 24hrs, if I'm not mistaken.