Sorry guys, but I personally never even use that function. So I've never paid any attention to it. I guess there is an option somewhere in the admin section that controls the duration...
Seriously, I always just scan down the left hand column of the forum listing and look for the icon that the forum has new posts in it. Then I click on the forum, open any threads that have new posts, and read those posts.
Always been the way I do it.........