You will billed for the hours that you used. Please find below links for reference.
"The prices shown are the monthly prices. How do I get charged if I use Managed Disks for less than a month?
The charges will be pro-rated on an hourly basis using the monthly price. For instance, Premium SSD for P15 is $38.013 (price may vary per region). For October, you will be billed per $38.013/(31 days x 24 hours) = $0.052/hour"
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