How often should tests be run in a continuous integration environment?

In a continuous integration (CI) environment, tests should be run frequently on each code change. This approach is fundamental to the CI practice, which emphasizes the integration of code changes into a shared repository as often as possible. Each time a developer submits a change, automated tests are executed to ensure that the new code does not introduce any regressions or bugs. This cycle provides immediate feedback on the new code's functionality and performance, allowing developers to quickly address any issues that arise.

Running tests frequently helps maintain code quality, reduces the risk of large integration problems later on, and fosters an agile and responsive development process. By catching errors early, development teams can save time and resources in the long run, since issues are addressed right after they are introduced rather than accumulating over time.

In contrast, running tests only at the end of development cycles, once a week, or only before major releases can lead to significant delays in identifying issues. Developers might find themselves dealing with multiple failures at once or face a daunting backlog of work before releases, which can impact the overall agility of the development process.