Just reading Jon's Blog post about swarm cells and already being concerned about a couple of hives trying to supersede, when is it too late?

The queen I have that came into lay in late july last year and raised the colony that came through winter is being superseded at the moment as is one of her offspring. Her other daughter has 5 supers on the go so I'm a little suprised to see supersedure cells turning up now.

When is it getting late in the season to see swarm or supersedure cells that stand a chance to build up?