[Mediawiki-l] increase session idle timeout

[fdo]

Hi
I want to increase the idle session time for my wiki users.
In my site, you must be logged in to edit pages and usually we're working in 
large documents so when you want to save changes (after several minutes) you 
got a "Please re login" due to your session has expired and you realise that 
it was a waste of time :(
How I can increase this time, is there a Localsetting.php variable for this?

Thanks