It depends a bit on the speed of your connection. I purchased it at the Apple store & used their fast wifi to download. As I recall, that didn't take too long to do. I did not install for awhile though as I had some other things that i needed to figure out 1st (I was running Parallels 5 which was not compatible w Mountain Lion so I had to figure out what to do w that--got upgrade). But then I also came back to Apple Store as I had decided to delete the Bootcamp partition which I had been using as a virtual machine in Parallels. they showed me how & that took about a min.
Then I started the install which I think is suposed to take about 35-45 min. Mine got all the way down to the "about a min left" when it hung. I know from my digital washer & dryer that the 1 min left can be well more than that so I waited & waited. Also asked the open training guy & he said to wait. Well we waited about 45-50 min & he checked w Genius bar & they said to stop it. So I think he had me power down & then power it. Then it started all over again. But this time it went through & finished.
Not sure why it didn't finish the 1st time. I'm glad I was in the Apple store to get their guidance, but was disappointed that the install took all the time so the open training time was over before I finished or right when I finished so wasn't able to get their help w learning some basics.
& yeah, it was $19 or $20.