And... drum roll...the answer is: at least 7 days.
They swarmed on Monday 26 July 2021 and, after a week of cool, wet weather, they flew off today, Monday 2 August 2021 - having inspected and then rejected 4 delicious bait boxes that I had set up to tempt them.
I was privileged to see them go - the first time I've been around at exactly the right moment. As advertised, they rose from the cluster, swirled around for only a few seconds, and raced off in a more or less tight cluster, making a bee-line for somewhere out of sight. I'll now be waiting for the panicked call from whoever's got them in their shed.