The developers of PHP has, in their infinite wisdom, decided that the default session timeout should be 24 minutes (1440 seconds).
This means that if you have a MediaWiki wiki and are editing a single page for half an hour and then click the save button, you are logged out and all your changes are lost. I just learned this the hard way.
Fortunately, you can change this with the
session.gc_maxlifetime parameter in
php.ini. So set this to a higher – more sane – value, such as 86400 (24 hours).