Active swarm cells, when does the hive normally swarm?

[D Coates]

I've got a nuc hive with a young overwintered queen.  The hive has expanded dramatically even though I've been stealing some frames with wet brood from this hive to act as an anchor for captured swarms and giving them additional empty drawn frames for added space.  It's now up to 20 frames via a nuc box with 3 5 frame boxes stacked on top of it.  While reviewing yesterday I found a frame with 7 queen cups hanging from the underside with young larva in them.  The cups were 1/2 the length of a full size queen cup and were about 1/4 full of royal jelly with a plump larva floating in the center.  I'd say they were all a couple days old but getting lots of activity.  I left them alone as I was not set up to split the hive at that point but I will this evening. 

This got me to thinking, is there a rule of thumb of when a hive normally swarms in reference to the age/activity of swarm cells?  Are they: any stage of larval development, about to be capped, any stage of capped, capped with with the top thinned, or there's no real rule. 

I'm under the belief it's once the queen cell is capped but figured I'd ask others with more experience.

[indypartridge]

Everything I've read says after it's capped and before the virgin emerges, but don't expect the bees to follow that as a hard and fast rule.

[yockey5]

swarming in imminent once the swarm cells are capped. You need to act now.

[FRAMEshift]

They swarm about the time the queen cells are capped.  We have on occasion found the old queen still in the hive shortly after a queen cell was capped, so it's not exact.

[AliciaH]

I'm throwing in the "about to be capped" option. 

[CBEE]

Sometimes I wonder what really sets them into the swarning mode. How quick could a hived swarm build up enough to swarm itself ?