How long can a Windows update sit at the same percentage?

Up to an hour is entirely normal on a large update, and the number tends to jump in steps rather than move smoothly. What matters is whether the computer is working. If the drive light flickers and the machine feels warm, something is happening even though the screen looks still.

Can I switch off while it says not to?

No. Under the words Do not turn off your computer, Windows is rewriting itself, and interrupting that can leave the machine unable to start. Wait it out. Only after more than two hours with no drive activity at all is switching off justified.

What happens if the update is interrupted?

Windows normally rolls back to the version you had before and tells you so once it starts up. Your files and programs are still there. The update is then attempted again at a later point.

Windows update stuck? How long you should wait

A Windows update can sit at the same percentage for a very long time, and that is usually normal. The number moves in steps rather than smoothly. The times it genuinely is stuck, the cause is almost always too little free space on the drive or a download that did not arrive intact.

The important part first: do not switch the computer off while the screen says "Do not turn off your computer".

Try this first

Wait, and wait longer than feels reasonable. A large update can sit at, say, 61 per cent for up to an hour. Allow two hours in total on an older machine.
Check whether the computer is working. Look at the small light on the front or edge of the machine, the one that flickers when the drive is in use. If it flickers, something is happening. A slightly warm case and an audible fan point the same way.
Keep the charger plugged in. A laptop running out of battery mid-update is one of the most common reasons an update truly locks up.
If nothing has moved for over two hours and the light is still, hold the power button for ten seconds until the computer goes dark. Wait half a minute and start it again. Windows normally rolls the update back on its own and returns you to the sign-in screen.
Give the computer room. Open File Explorer, click This PC and look at the C drive. Windows needs at least 20 GB free for a major update. Free space up through Start, Settings, System, Storage, Temporary files.
Try the update again. Settings, Windows Update, Check for updates.

If that did not help

Run the built-in troubleshooter. Settings, System, Troubleshoot, Other troubleshooters. Find Windows Update in the list and click Run. It resets the download for you and takes a couple of minutes.
Restart and try once more. The troubleshooter usually makes no difference until the computer has restarted.
Change network. If the computer is on Wi-Fi and sits far from the router, the small box with lights that gives you internet, try a network cable or move closer. A download arriving in pieces is a common reason the same update refuses to install.
Unplug the extras. Printers, cameras, USB sticks and external drives. It is uncommon, but they can hold up an installation.
Fetch the update by hand. If Windows shows a code beginning with KB, such as KB5062553, go to catalog.update.microsoft.com, search for that exact code and download the x64 version, which is what almost every computer uses. Open the file and let it install. Never download an update from any site other than Microsoft's own.
Check the date and time. Settings, Time and language. If the clock is wrong, Windows cannot verify the download. Turn on Set time automatically.

Before trying anything that resets Windows

Put your photos and documents on a USB stick, an external drive or in OneDrive first. A reset that does not go as planned can take everything on the computer with it, and that step cannot be undone.

Find out your Microsoft account password before you begin too, since you have to type it in yourself once the computer comes back. Never give that password to anyone, including us.

If the computer ends up in a loop where it restarts over and over, shows a blue screen after the update, or stops on the same code three times running, it is time to ask a person. There are commands that repair Windows without touching your files, but they should be run with someone watching the screen alongside you.