mean_time_to_happen gives the average time it takes for the event to fire. Set it to days = 1 and it will fire very quickly; set it to years = 60 and it will take a long time to fire. You use factors to increase or decrease the likelihood that it will fire, factors greater than 1 will slow it down, factors lower than 1 will speed it up. A factor of 0 will disable it. (Do not use negative factors.)