For each task, CircleCI rounds up to the nearest second, then to the nearest credit. In practice, the type of resource class you are running controls your credit::time ratio.
For example, a Docker executor with a medium resource class costs 10 credits/minute or 1 credit/every 6 seconds. Since CircleCI will round up to the nearest second, then nearest credit, it would be safe to say that 1 credit is consumed every 6 second block.
More specifically, let's consider running a task for 10 seconds using a Docker executor with a medium resource class. Since this is not a fraction of a second, we don't need to worry about rounding up to the nearest second. However, it would be rounded up to the nearest credit. Therefore, with this example, a 10 second task consumes 2 credits.
You can view a list of executors with corresponding credits per minute here: https://circleci.com/pricing/#compute-options-table