Time required for updating

Well thanks for the input but there are so many variables it is impossible to accurately estimate the time of a download and install. If you feel differently you can tall Apple at www.apple.com/feedback.

There may be an issue with your network, it's bandwidth or what you have installed on your computer. Network speed is only 1 item it has nothing to do with the bandwith of your network or what other devices are competing for bandwidth. In addition, what you have installed on your network can have a direct bearing. I live in an area that gets high speed networking, my download speeds are 56GBS and it take my system approximately 45 minutes to an hour to download and install Mac OS updates. So perhaps the issue is with your system or the network you use.

If you want to post an EtreCheck report we can look for obvious issues with your installation.

There is no way to accurately predict your internet connection speed which may fluctuate greatly. Everyone has a different way to connect (wired/wireless) with wildly differing speeds - how could anyone have the "ability" to accurately predict?

Mine is a slow connection and I know from experience that a +/- 5 GB download will take at least an hour to an hour and a half. The time displayed is usually quite accurate - maybe because during such a download, no one else is "allowed" to use the internet so as not to use up all of the bandwidth which I need for that big download.

Downloading Big Sur took way longer than 2 hours @ 12 GB size (actually about 2 hrs 40 minutes).

After that, there is the "preparing" and other pre-install processes to deal with.

No, all times are approximate. Just do it late at night before going to bed, then you can sleep on it while it's doing it's work.

Network performance is an integration problem of all of the different bandwidths that the network hops to you allow. Installation time is Apple applying its hand at prestidigitation, and there is no finite time that you can count on. Differences in hardware configuration can impact installation times too.