I used to work for a company who's core application, I was told, had over 40,000 unit tests (no joke). I asked them if they had a CI process and ran those tests at every build. The answer I was given was "no" because "running a build with that many tests
takes way too long to run every day, much less at every build".
It’s too bad their application wasn’t developed in .NET using Visual Studio 2010 so that they could take advantage of Test Impact Analysis. This new feature analyzes the code changes developers make and selects only the unit tests that have been impacted by
the code change. The developer is free to choose between a subset of tests (those that need to be run based on the code changes made) or run the entire suite of tests for all-up testing. This short video will give you an overview of Test Impact Analysis and
show you how to configure it in your Visual Studio 2010 Solution.