how long should it take for the operating system update to install when starting up a new iphone?

Over the air update depends on the internet speed, the iOS version and the existing iOS versions. At 15 Mbps for me it took 30-40 mins to update iOS 14.2 from iOS 14.1

iOS update through iTunes will depend again upon the internet speed, the new iOS version and the existing iOS version and over and above the Storage of iPhone for backing up before the update.