Are you sure that the hive didn't swarm? How long between the first and second time was it that you waited?
As for timing of killing off the old queen, in some cases the workers don't bother to do it at all. They let the new virgin take care of that job. However, sometimes she doesn't do it either and you end up with a two queen hive.
I know from what i've seen on my hives, if the workers are getting rid of the old queen, they will wait till a couple days prior to the new virgin emerging from the cell before they kill her off. I guess they wanted to get as much out of her as they could. lol
As for timing of killing off the old queen, in some cases the workers don't bother to do it at all. They let the new virgin take care of that job. However, sometimes she doesn't do it either and you end up with a two queen hive.
I know from what i've seen on my hives, if the workers are getting rid of the old queen, they will wait till a couple days prior to the new virgin emerging from the cell before they kill her off. I guess they wanted to get as much out of her as they could. lol