As of April, Apple's Mac App Store was in existence for about a year, and it was already boasting 10,000 apps. In late July, Mountain Lion was among them, with eyes staring sharply as if to ask: "Did you order a memory upgrade?" ... because you're going to need at least 4 GB to run it.
By the way, most estimates that we came across of the total time required to install Mountain Lion topped out at about an hour. Clearly, that didn't include the download, which alone took about an hour. We suppose one could download the 4.3 GB file once and move it from machine to machine, but that runs afoul of Apple's single-use licensing agreement. Allow an extra hour of download time for each machine.