Manual:$wgExtendedLoginCookieExpiration

Details
The purpose of this cookie is to allow having a different (usually longer, but this is not enforced) duration for "" login-related cookies.

This means you can e.g. set the standard cookie duration (widely used for MW cookies) to 30 days, but set the "" duration to a year.

Logins where the "keep me logged in" option is not checked are entirely unaffected by this setting.

Logins where "keep me logged in" is set use a mechanism that can recover from server-side session expiry, so the lifetime of the extended login is unaffected by.

The values mean:
 * null - MW will follow  instead.


 * Positive integer - Duration of "" sessions, in seconds.
 * 0 - "" sessions will last until the browser closes.