@Subarna Mukherjee Post 750 hours of usage it will be charged.
For more information, you may refer to the below-mentioned articles:
Check the usage of free services included with your Azure free account
Azure Account FAQ
Avoid charges with your Azure free account
If you still find any difficulties, please contact billing and subscription management support, It's Free and the billing team provides more specialized assistance and detailed information on your query!
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
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