Operating System compile time

Ales Holecek, vice president for development in Windows team said that it takes about 16 hours to build Windows 10. And that it's built automatically every day during the night.

It's not built using single machine, of course, but rather using some build farm.

Third-hand information I have is that it takes about a day to complete a Windows build. Which is more or less in line with attempting to build your favorite OSS Operating System from scratch.

Building a modern operating system is a complex and difficult task. The only reason why it doesn't take longer is because companies like Microsoft have build environments setup to help automate integration testings. Thus they can build a system with less manual effort than is involved in most OSS builds.

If you've like to get a feel for what it takes to build an operating system, might I recommend the free eBook: Linux from Scratch

For a more automated build, try Gentoo. Both options should give you a better idea of the Operating System build process.

OP is asking about Windows:

"There are no other software projects like this," Lucovsky said, "but the one thing that's remained constant [over the years] is how long it takes to build [Windows]. No matter which generation of the product, it takes 12 hours to compile and link the system." Even with the increase in processing horsepower over the years, Windows has grown to match, and the development process has become far more sophisticated, so that Microsoft does more code analysis as part of the daily build. "The CPUs in the build lab are pegged constantly for 12 hours," he said. "We've adapted the process since Windows 2000. Now, we decompose the source [code] tree into independent source trees, and use a new build environment. It's a multi-machine environment that lets us turn the crank faster. But because of all the new code analysis, it still takes 12 hours."

SOURCE

Also see Mark Lucovsky classic presentation on developing Windows NT/2000.

I don't work at Microsoft, so I don't know for sure...