As Tekserve noted, those models rely on SSDs of various sizes to store data, applications and the operating system.

Machines with traditional platter-based drives, including the newest 13-in. and 15-in. MacBook Pros, and the two iMac models, required much more time to process the upgrade, ranging from 40 minutes for the 21.5-in. iMac to 57 minutes for the non-Retina 15-in. MacBook Pro.

Reviewers reported different upgrade times. Michael deAgonia, who , said an upgrade to a SSD-equipped 15-in. MacBook Pro took approximately 35 minutes. -- like Computerworld, a site owned by IDG -- said his stopwatch recorded 15 to 25 minutes, depending on the Mac.

, or The Unofficial Apple Weblog, reported even-longer times of 30 to 50 minutes.

The Mountain Lion upgrade is a hands-off affair once the user has downloaded the 4GB installer from the Mac App Store, allowing Mac owners to step away from the machine and come back when the new OS is installed.