The default is for the forums to log you out, but you can change the default:
1. Log in
2. Click "User menu" (top menu option to the right of "Mark")
3. Click "Edit your preference"
4. Scroll down a little to "Remain logged on when you return to use the forum at a later time?" and change the setting to "Yes"
5. Scroll down a little more if you want to use "Mark" - look for "Use MARK time stamp feature?"

Note: you can also set your preferred time zone here along with several other settings.