I browse FT in Chrome on Windows 7 and 8 and have been logged in without interruption for at least the past few months. Looking at my flyertalk.com cookies, "bbuserid" and "bbpassword" are set to expire 365 days after they were placed. If everything goes as planned, I should only log in once a year.

Is this happening on a mobile device or a desktop device, and what type of browser are you using to access FT? I wonder if your browser is having trouble storing your FT cookies.