Is it normal for the site to log you out after a period of inactivity?

After a time, I noticed that if I'm logged in and I leave a page in FF alone for maybe five to ten minutes (I tend to multitask online), when I change page/refresh/try to post/etc., I have to log in again.

This isn't too bothersome, but I'd like to to know if this is normal behavior, and if not, if there's something I can do to resolve this minor quibble.

Thanks again!