Hello hanspumpel
You can extend PHP session time to 1 hour for example.
You can edit the php.ini file:
session.gc_maxlifetime = 3600
Or add this line to your config.inc.php file:
ini_set( 'session.gc_maxlifetime', 3600 );
I will add this setting to the recommended settings (installation instructions).
I also think there is a misunderstanding:
Typing in a TinyMCE rich text field does not count as activity for the server. This will not maintain your session because this typing activity is only handled by the browser. No request is sent to the server during this time.
If you want to send a request to the server while typing, you can for example open a menu link in a new tab.
Do this answer your question?
-Edit-
The default PHP session timeout is 1440 seconds (24 minutes).