When beeing logged in and writing a new mobiki page or changing an existing one, mobiki starts a session. While beeing logged in each time a webpage is loaded the session gets updated. For security reasons you get automatically logged out after not using mobiki for a specific time. This even can happen while editing a page without intermediated saving for a longer time.
The default timeout for PHP sessions is set individually for each PHP installation, the default ist 24 minutes. So if you write a long article, just do some intermediate saving, don't forget it.
Configure your Webserver
If you are the owner of the webserver, you can adjust the garbage collector, which removes timed out sessions. Just increase the following parameter in your php.ini, session.gc_maxlifetime(). The value is set in Seconds.
session.gc_maxlifetime = 1440
(1440 seconds = 24 minutes)