You can set this time limit in your board's settings. It's a standard feature.
I know I can set it. Globally. I want to be able to set it per user group. Or disable it per user group.
Sorry, I had "sleep in the eyes". :-[
it was already asked and here is the solution http://www.elkarte.net/community/index.php?topic=2389
and here the addon http://www.elkarte.net/community/index.php?topic=2407
Just to avoid duplicates, I'll move this to the "exterminated" board since there is already a topic about it. ;)