The granularity of the changes depends on the hour.
From the AWS pricing site http://aws.amazon.com/ec2/pricing/ :
The price per instance-hour consumed for each instance, from the moment the instance is launched until it is completed or stopped. Each Partial instance used in the watch will be billed for the full hour.
If you do not calculate the time that is under the threshold for the free level, the second you use an instance of EC2, which you charge the entire hour. If you go one second in the first hour, you pay for the full second hour.
One caveat: instances of Spot.
If random instances are interrupted by AWS (not you), before you start using the full hour, you will not pay at all. If you interrupt a spot instance, you pay for partial hourly use (this is the full hour, rounded off by instance on request).