The 12 hours here is the duration that you have an EBS volume (with provisioned IOPS) available to you; meaning from the time you create the volume up to the time you delete/release the volume. It doesn't have to do with EC2 instance power state.
The point here is that:
... Provisioned storage and provisioned IOPS for io2 volumes will be billed in per-second increments.
Do I get charged for stopped instances?Accepted Answerasked 5 years ago
Why my account was charged using RDS?asked 8 months ago
I changed the storage type of a RDS instance from gp2 to provisioned IOPS after running out of IO Credit. Do I have to wait to finish the Burst duration?asked 9 days ago
I tested Snapshot of 60gb (10usd) light sail instance. Its actual size is only 6.2 gb. How much i get charged? 6.2Gb or 60gbAccepted Answerasked 9 months ago
Do i get charged for Reserved Capacity if the machine is off?asked 5 months ago
Why is there a cost of IOPS-month provisioned for activated ebs on stopped ec2?asked 7 months ago
How the Detailed monitoring is actually charged.asked a year ago
How is Provisioned IO charged?Accepted Answerasked 2 years ago
Charged $1.00 After Creating An Instance In EC2asked 9 months ago
Will I be charged while the EC2 is in stopping status?Accepted Answerasked 7 years ago