Apologies to everyone who has experienced this problem, especially glock20rocks. 2 hours is completely unacceptable
It has more to do with the length of a session. When you sign on, you start up a session, which gets stored in the phpBB session table. To make it fast, it uses the MySQL table type HEAP (table is created in memory). This table is far smaller than the normal MyISAM MySQL table. The default session timeout length is an hour, which means the row will sit in that table for about an hour before it's deleted. SO, we could hit that limit in an hour, even though not that many people are currently logged on. There are also ways to have multiple entries for yourself in the session table... I'm not quite sure how this affects (or is effected by) people who check the "Automatically log me in" box.
We've just implemented a fix for this problem. If this error occurs, it will automatically delete the oldest 50 sessions. If somehow you happen to be 1 of those 50 (walked away for a bit, typing a novel for the forums, etc) all that means is you'll have to log back in again. So I guess if you ARE typing a novel, better copy your post before hitting submit!
If, even after this fix, you get this error repeatedly, and it takes more than 15 minutes to finally get in each time, please let me know, and we'll look into a more aggressive solution.