Evil Genius FAQ

Time clocks:
  Several rooms can have time clocks; any hotel room, the infirmary, control 
  room, armory & mess hall.  They let you shut down the whole room (by setting
  them to the Red square), making all devices in the room consume 0 power. 

  Other settings adjust how minions will staff any objects in the room that
  require people to run.
  
  If you set it to 'Yellow', minions will staff it if they have nothing else 
  to do, and immediately leave if you ask for an object to be moved, room
  built or anything else.

  If you set it to the first green square, you want the room to be staffed at
  all times.  However, when a minion gets tired and leaves, it may take a while
  for another minion to come in to replace him.  This is fine for infirmaries
  and the mess hall, but for the control room you'll experience mission 
  failures and minions unable to steal/plot in the time it takes for someone to
  leave and their replacement to come in.

  It looks like every green square after the first results in 1 extra minion
  who will stand around in the room waiting for another minion to leave.

  Note; since this only affects objects that require staff to run, it doesn't
  seem that using any green setting has any effect on rooms with no staff-run
  objects; armories without security desks, lobbies without pianos or desks, 
  etc...