I think that you will find that when logging in there is a box that says something like, "remember me". Tick that box and you won't time out, unless it's something to do with your ISP.
Your login will just be there each time you return, unless of course you log out. Maybe I don't understand it right though, because I only reboot my PC every couple of months or longer.
If I use this terminal, a PC at home, then the forum is always there.
If I use my netbook from a friends house, nine miles away, then I have to log in each time I visit, and it "forgets" me each time I leave the forum to go to a different website.