Is it possible that users are able to modify the user online list as in IPB 3.0? Currently it seems that the cut off time for users viewing the forums is 30 minutes and cannot be modified. 

Rikki said this on the preview board: http://ips41preview.invisionpower-staging.com/topic/287-online-list-timeout/

No, you can't do this on 4.0 because we use PHP's built-in session handler, so how long their session lasts (and therefore how long they show as 'online') isn't determined by us unfortunately.

You could replace the list with this from the marketplace

It is a free one