Login cookie duration/update?

How long are the cookies that retain someone's logged-in status supposed to remain valid, and why are they not updated/refreshed if a user comes back in via effectively the same login session to give another duration-period's worth of validity? It's annoying AF to log in with "remember me" set, do some edits over the next few days, and then suddenly be in a logged-out state only a few hours after the last time we accessed the website. Cookies carrying a login state should refresh to a new duration any time the user comes back in using what existed already, because those implicit credentials are inherently self-authenticating and we shouldn't be ignoring that fact and ticking the clock against that user with no regard for prior activity.

_H*