Thread:Project:Support desk/MediaWIki Session idle time

in one mediaWiki project it happens to me that after a user has been idle for some period (like 20 minutes) he gets automatically logged out.

I have already tried to set in LocalSettings.php $wgCookieExpiration = 60*60*24*365; and $wgCookieExpiration = 0; but it does not seem to work, I still get logged out.

I have tested this in Chrome and Firefox.

Any ideas ?