how long does it take ios 16.4 to update

For reference, I have a 3rd gen iPhone SE that I just updated to iOS 16.4. I have a "slow" DSL Internet connection of around 40 Mbps download, and the whole update process took about 15-20 mins.

Downloading the update depends on the speed of your network and the load on the servers. Usually it is "extra busy" when a new update is released. When the update is downloaded and starts installing, it usually takes somewhere between 5 and 10 minutes before the phone is restarted after the update and ready to use again.