I did a 2nd Gen ATV and I'd say those times were similar to my experience. However, since I am a bit of a tinker I tried a couple of different things.Here are the timings for a 2nd gen I just did:
09:00 AM. Download start.
09:02 AM. Step 1 of 2 preparing update
09:20 AM. Rebooted itself
09:21 AM. Apple logo progress bar appeared
09:24 AM. Step 2 of 2 applying update
09:35 AM. Rebooted
09:36 AM. What's new screen appeared.
I think the 3rd gen took a bit longer.
I downloaded the 5.3 firmware and used iTunes (i.e. micro USB cable) just see if I could downgrade. Not only did it work the whole process finished inside of maybe 3-4 mins.
Surprised by the speed, I wanted to see how fast it would take to upgrade since the normal method took so long. So I downloaded the ipsw of the 6.0 firmware and upgraded it using iTunes again. Same thing -- maybe about 3 mins or so.
After that (since I knew I could downgrade, if necessary) I decided to upgrade my 3rd gen ATV (which I had been holding off on). Seeing the speed advantage and having read about how many people were getting bricked ATVs doing normal upgrade I just upgraded it by iTunes as well. Zip, zoom, presto it was updated and ready to go just as quick as the 2nd gen ATV.
Now granted I had downloaded the ipsw image file beforehand saving a few mins of download but, by all means otherwise, if you've got the micro USB cable i'd recommend upgrading your ATV using iTunes, as well. Save yourself a whole lot of time.
Edit: quick update, make sure you are aware that upgrading by iTunes is a wipe and restore -- not a settings preserving "upgrade". You will have to setup your WiFi, HomeSharing / iTunes, and enter your Netflix / Hulu account passwords again.