Manual:$wgExtendedLoginCookieExpiration/en

Details
The purpose of this cookie is to allow having a different (usually longer, but this is not enforced) duration for "$1" 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 "$1" duration to a year.

The values mean:
 * null - MW will follow $1 instead.
 * Positive integer - Duration of "$1" sessions, in seconds.
 * 0 - "$1" sessions will last until the browser closes.