You will not be charged for workflows that are in a hold state waiting for manual approval. Since the hold state doesn't require any resources, there is no charge. More information on credit charges can be found on our pricing page -- in addition, we have some further details in our documentation.
One note to the above is that if you had jobs that ran before the approval step those jobs would have consumed credits to run. However, while the workflow is in an "On-hold" state, it doesn't consume credits.