Manual:$wgCookieExpiration

Details
The default value is 30 days (30 * 86400 seconds). This means most MediaWiki cookies will last 30 days. This includes login/session cookies when the " " option is not set. When " " is set,  is used instead (although by default that settings falls back to  ).

When " " is not set, login expiry is also limited by  (the session expires when there is no interaction for that time span).

Since MediaWiki 1.22 setting this configuration variable to 0 makes cookies session-only. This was previously documented as working, but broken.

This is used by WebResponse::setcookie and mw.cookie.set, though an individual cookie can have a custom expiration.