how long can it take for an update to be ready?

The update will take as long as it takes depending on the speed of your Internet connection. If you have updated in the past, the current update should take about the same amount of time as previous updates. For me, that's about 12-15 minutes on a 200 Mbps Internet connection.

Apple logo with progress bar after updating or restoring iPhone, iPad, or iPod touch - Apple Support

Take a look at the info in that link

No. The same factors mentioned matter. No one can give you a real answer to how long it will take. I suggest waiting until you have time because you have no way of knowing how fast or slow it will be. Allowing it to do it overnight while you are not using it is an option.

Depends on if the servers are bogged down and your internet connection. Sometimes can be fast and sometimes can be very very slow. Time of day may matter due to the number of others accessing at the same time.

It all depends on the speed of your Wi-Fi Internet connection. Normally, an update takes about 12-15 minutes here.

Move the phone as close as possible to your Wi-Fi router and avoid using other devices on the network until the update completes.

Is the white bar under the logo moving?

Location doesn’t matter. The fact that it’s there does. What you have been told already is still true. It can go fast or very slow.

Yes it can. Like I said repeatedly it can take a very long time or go fast. Honestly just depends. As of now you’re not seeing it loop or anything. So it’s very likely just going very slowly for you.

Accidentally replied to you. Sorry