I've seen a couple of projects using CI to build the project and run their test suite.
All of them have been interpreted languages like PHP, Python and Ruby. Their builds and tests took between 30-45 minutes. As far as project size and complexity, these were projects that were built and maintained by four person teams over the course of 3-5 years, so it's not like they were massive services with hundreds of developers.
I'm still kind of new, I worked at a couple internships and I've been working full time for a year so I might be totally wrong but I feel like these CI pipelines could be optimized to run faster.
on travis for a repository that builds 14,479 objects, 67 libraries, and 456 static executables, 284 of which are test executables which are run too. If I want to run all the test binaries on freebsd openbsd netbsd rhel7 rhel5 xnu win7 win10 too, then it takes 15 additional seconds. On a real PC, building and testing everything from scratch takes 34 seconds instead of two minutes.