About 20 minutes seems correct..
...but, as for the updates/security software... that number is inflated based on your internet connection speed, not time-to-disk. I'm guessing you knew that, but for me, that is the time-cost of the initial system. Every subsequent 'build out" suffers only a fraction of that time, as I note the updates, and create a more efficient method for those factors/installations/patches. (PXE installs suffer very little past a good build serve).