you can select how long you will stay logged in when you are entering your username and password.
just log out, switch to "forever", and log in again
I think he is asking about
total time logged in count.
I know there's a way to do it, as I once did it with an old account.
I've just looked through the settings loads of times under "PROFILE" for a way to set the length of time I stay logged in, and can't find it.
Use auto refresh for every 10 minutes, you'll be OK. I think forum logged-in timer cut down in 15th minute.