How long does a ZigBee firmware update take?

Nymea correctly identifies new firmware updates for several of my gadgets. When I start an update, the status information (applying update) is shown right away. However, the update progress remains in all cases I have been able to observe at 0% and even six hours after starting the update is still being applied and the progress is stuck at 0%.
Yesterday, I tried to update an Aqara TVOC Air Quality Monitor and an Ikea button. Both are reasonably close to the ZigBee coordinator, an ConBee 2.
So, how long is it supposed to take?

Update: At the fifth attempt the TVOC Air Quality Monitor has managed to update its firmware within about one hour. It’s hard to say with any certainty as I see no log entry which says when it ended. I noticed that something must have happened when I saw that the display of the TVOC had reverted to its factory default. The details view for the device now shows the new software version, so all’s well, it seems. Two more devices to go.

Normally it should take about 20 minutes to update a ZigBee device over the air. If the progress stays at 0, the device doesn’t seem to start fetching the firmware file. For battery powered sleepy devices it may help to wake them up by pressing a button right after starting the update in nymea. I’ve seen devices wait for a while until they actually start. Yesterday I’ve tested this on a hue smart button to verify this report. It did stay at 0 for a while but then I had to leave. When I returned home the update was completed.

Thank you, Michael. This did it.
The Aqara TVOC Air Quality Monitor does indeed wait for a button press to start updating and the update takes something on the order of twenty minutes. While updating it shows an icon on its display which rather looks like an empty battery. This a bit disconcerting. After updating it resets the display to the factory default and all seems fine. All my Air Quality Monitors are now on the most recent firmware.