If you have a problem with getting logged out of the forum often here's how to fix it.
Log out of the forum. Now start to log back in but make sure you check the remember me box. If you have the remember me box checked the forum will put a cookie on your computer which will keep you logged in no matter how long you are inactive.
This may not solve every issue with being logged out but the vast majority of people with this problem do not use the remember me box to stay logged in and that will make your login subject to the forums default idle timer.