With EFS, throughput is billed per month, and it's based on the average throughput provisioned in excess of what your EFS Standard storage allows for the month. Your throughput is measured in "MB/s-Month," which are added up at the end of the month to generate your monthly charges.
Note that AWS billing works hourly. This means that if you enable provisioned throughput only for a certain number of hours, you will be charged that pro-rated amount only for the hours / days you used it for.
Take a look at more information on the EFS pricing page: https://aws.amazon.com/efs/pricing/ and in particular, check the Example 3 at the bottom of the page where you can find other pricing examples.
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