It depends on how you set your cookies - if I close all my browser windows containing AO, or am not active for a certain length of time (i.e. I don't load any new AO pages), I'm automaticaly logged out (and, presuming Negative has similar settings, that's why he'd need an auto-refresher, although it's fairly easy to implement one in JavaScript). But then, I'm paranoid about security.

It would be interesting to see how long people were online for, but unfortunately the stats would be twisted somewhat with people on always-on connections staying logged in 24/7.