How large of a download is this? after the file is downloaded how long will it take to install/update?

It depends on how fast your Internet connection is. How many megabits can you download per second? The download is approximately 12.5GB. It took me about 6 minutes and 18 seconds to download. Gigabit Internet Service connected with Ethernet not Wi-Fi averaging about 930Mbps.

time softwareupdate --fetch-full-installer --full-installer-version 11.4
Scanning for 11.4 installer
Install finished successfully

________________________________________________________
Executed in  378.07 secs      fish           external
   usr time  229.50 millis    0.08 millis  229.41 millis
   sys time  123.03 millis    1.03 millis  122.00 millis

If you have poor Internet speeds find a location you can go with a fast connection and connect with Ethernet if you can.

The time to install operating system takes about 60 minutes to install and fully patch on an SSD. If you have an HDD it will take longer.

And,even though the installer is 12.5 GB, it requires a minimum of 35.5 GB or 45 GB of empty space on your hard drive (depending on the OS version you are upgrading from) to expand and install. What year/ model is your device?

And, FWIW, at my house, it takes 1.5 - 2 hrs for a 12 GB download.

This is an upgrade and not an update. If your profile is correct, you will need 45 GB of available storage on macOS Sierra before you start the download. Have your MacBook Pro connected to a power source before you even consider starting the installation download. Don't attempt this upgrade on battery alone.

You should have already verified with the free Go64 which applications that you currently have that are 32-bit and unsupported on Big Sur. This may mean obtaining 64-bit Big Sur compatible vendor updates, and some vendors are charging for this update. You should also verify that your printer/scanner product has vendor driver support on Big Sur, or that as a networked device, it is supported by Apple's built-in AirPrint. And be certain to have a last Time Machine backup of the current operating system before you commence the Big Sur upgrade.

Thoroughly remove all anti-virus, so-called cleaning applications, and any application that has installed kernel drivers before the upgrade as Big Sur installs on a read-only partition and. you won't be able to remove old kernel drivers afterward.

To summarize, a download such as this when done via a wired Ethernet connection will typically take up to an hour, more or less. Then the installation can take up to 45 minutes, more or less, with several automatic restarts.

As the previous posting have mentioned .

Upgrade to Big Sur will require a quick, stable and reliable Internet Connection - preferred method is via Ethernet Connection.

If not available and using Wifi - move computer as close to the Access Point / Router and disconnect all other devices connected to Wifi. Want the maximum bandwidth for the computer being upgraded to Big Sur.