Your login is sustained by a cookie held on your PC, if your IP hasn't changed since the last time you were active (clicking any link on the site) then you shouldn't have to login more than once a week (cookies expire after 1 week).
The forum does have a secondary security measure. If your IP has changed since you were last active (either you logged onto a different PC or your internet connection reset) then it will ask you to re-login.
A while ago (although I haven't heard anything from users recently) we had a spate of attempted logins by people wanting to take over our user accounts. But the affected members were presented with a "catpcha" screen after "too many login attempts". I presume this isn't the case here?
Another cause could be a corrupt cookie .... clearing it first, then loging in again may help.
Keep me posted.
