How long does an update take?

It's very different. It depends on how busy the servers are, just as it is about how fast your WiFi connection is.

A tip if your update has stalled.

First, reboot your phone. Turn it back on and go to:

Settings -> Display & Brightness -> Auto-Lock. Select Never.

Remember to put it back after updating. The reason I do this is because the screen does not turn off while downloading the update.

After that, connect a charger and of course make sure you are connected to WiFi, then try updating again.

Also depends on what version you were updating from and to. If you were updating to iOS14 from 13 or 12, that will take longer than, say, going from 14.4.1 to 14.4.2.