Manual:$wgCookieExpiration

Details
The default value is 30 days (30 * 86400 seconds).

This means most MediaWiki cookies will last 30 days.

This includes, "keep me logged in" cookies, unless  is used (which defaults to 180 days).

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.