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…