Is leaving my tab open but not playing anything for 12 hours while I have dinner, go to bed, shower, considered "online"?
well, most would agree this should not be counted as online when using the data for this aplication.
but even not playing anything for 1 hour would be enough to be considered offline (or you could also say "x users active"), since like aphuey said theres already data gathered and processed about this.
and since subsonic counts long songs, eg mixes that are 30 min long as "user has played song x 30 min ago" this would also be practical even for mixes (under 1 hour mixes only i think.
and as it stands, the "user is playing song x minutes ago" goes away after 60 minutes. having a little text somewhere (with option to disable it in settings) which sums up every user that is still shown in the "active pane" would not be hard.
i thinks its more a design problem, since this would kind of show duplicate stats. maybe an option for admin to either show the small stats version ("x users active") or the big one (well known^^)
think small screens and phones, maybe that would be practical, or maybe just a list of names instead of the "active users" pane without the current song and cover (again, selectable in settings).
logging out should not be enforced just to gather statistics, as its really a pain to constantly log in again.