Manual:$wgCookieExpiration

Details
The default value is 180 days (180 * 86400 seconds/day). This means MediaWiki users will be logged out automatically 180 days after they log in if they select the "remember me" checkbox when logging in.

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