The estimate is shown in the Windows Update screen as well as via the Start Menu. The author tested the estimate's accuracy with a timer and concludes it was too pessimistic:
I used a stopwatch to time the installation, and it only took about a minute and ten seconds for the update to install, and boot to the home screen. I think that's very impressive. Sure, the ETA of 5 minutes was wrong. But that's fine, it's probably a maximum (or average) that Microsoft estimates, that the computer needs to install the update.