Hi, yes, Lara gave you good advice

There is a default timer built into the software that will log you out if it detects inactivity for a period of time. This protects accounts from being hacked into if members accidentally forget to log out, especially on public computers. The software does not recognize typing replies as activity, only sees clicking around as such, so if you are working on a loooong reply, it sees you as inactive
But....
by checking that "remember me" when you log on, it overrides the default and you will only be logged out when you actively do so.