The forum session length is currently set to 3600 seconds (1 hour), if you tick the 'remember me' box the board will remember you when you return within a 1 hour time span. During a period of inactivity exceeding 1 hour your session will automatically expire and you will be logged out.
However if you login, read a couple of posts and return before the session expires you will be automatically logged in. If you login just before your session expires it will be renewed for a further 1 hour (a bit like visas really ).

Some boards set the session length to a higher number i.e. several hours, days or even weeks, however this increases load on the server which I'm trying to avoid. Also a lower session length is an added security feature if you forget to log out at an internet cafe.

If you do as Bob suggests i.e. click remember password when logging in and un-tick this from the Crap Cleaner program, then all you have to do is click log in without the need to manually type your username and password each time. It works for me OK.

If you're able to view new posts each session, and is remembered each time you log in then your cookie settings are working fine.