How long should it take to download OS 16.4 on a new iPad?

Assuming that your device has adequate free space (4GB+ recommended), the time required to perform OTA system software or App updates are principally determined by three factors:

• the size of the download(s)
• the available bandwidth of the network/internet connection
• server load (noting that Apple’s servers are regional)

Larger downloads, or poor network connection bandwidth, will result is slower update performance.

Speaking of personal experience of updating many iPad and iPhones from iOS/iPadOS 16.3.1 to 16.4 - with a fast internet connection - the iPads required about 15-20 mins to download, unpack, verify and install the update. The iPhones required somewhat longer than iPad (this being a consistent observation across all updates) requiring about 30 mins to complete the entire process.

If the update is performed using a Mac or Windows PC, the actual device update is somewhat quicker - as the installation package is downloaded, unpacked and verified by the host computer.

Be patient. If the process doesn’t complete within a few hours, something may have stalled - and a restart may be required.

You mean the over the air update? I think that's less than 1 GB depending on the build (for specific iPad models). It can be somewhat slow if your connection is slow or there's some other congestion at the server. I've seen it really slow before, so that's possible. But it should theoretically take less than 10 minutes for the over the air update.

I did mine a few days ago with my Mac. Can't be sure how long, but the download part was less than a half hour and it was more than 5 GB.