there was a thread about this somewhere. i believe i read that events happen every 24 hours of actual game play. i am not sure the exact definition of game play. does it include the time the game is off? does it include time game is paused? is it affected by half, regular, and double speed? perhaps those variables are why you have not seen an event.

another theory is perhaps the events are happening but you are not seeing them. perhaps they happen when the game is off.