Credits are counted per test, not per minute, hour or engines. Please note that each test takes a couple of minutes to start up and spin down. An hour long test will runs approximately 55 minutes long, plus time to start up and shut down.
1 test credit will be deducted from your test credit balance once the test is completed. Multi tests will be deducted 1 credit as well.
Note: Max test duration is different based on the plan selected.
If for whatever reason a test failed to run (for example, an error in a user uploaded script) or generate a report (tests need to run for at least 2 minutes to generate a report) the test will not be deducted from your test credits.