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. Setting this configuration variable to 0 makes all cookies session-only.

The previous value of this configuration variable was 30 days (30 * 86400 seconds/day). The value was raised in.