you can actually do a lot.. if you have the know how with coding.. lol I don't
See with the online.php file it tracks what they are doing for the most part and where they are.. not really for how long or anything. I mean I do believe it puts a timestamp on their last activity.. like replying or viewing a page.. but it pretty much ends there.
I am sure you can add a function, and a couple of fields in the user table.. and it can track current online time, and total online time for ya..