To determine how often to parallel test, analyze your existing test regime, break it down into logical units, and reorganize it for fully automated parallel testing. Identify tests that can be done in parallel and those that cannot or should not be done in parallel, scheduling the latter as needed. Parallel testing is now the standard, replacing sequential testing and manual testing, offering significant advantages in terms of time, scope, and cost. It is essential to consider when parallel testing will be more productive and cost-effective than sequential testing, allowing you to optimize your test regime for maximum efficiency.