Online Users block settings
The Online Users Block is configured from Administration -> Configuration -> Blocks. Click on Settings to see the configuration page.
There is only one setting to change: block_online_users_timetosee which is the number of minutes after which users will not be displayed in the list. The default is 5 minutes. You may want to adjust this if the list of users displayed is too long or too short.
Note: If separate groups are enabled in a course and this block is added onto the course page, only members of the group will be displayed in the list.