A custom metric can be one of the following:
Standard resolution, with data having one-minute granularity
High resolution, with data at a granularity of one second
By default, metrics are stored at 1-minute resolution in CloudWatch. You can define a metric as high-resolution by setting the StorageResolution parameter to 1 in the PutMetricData API request. If you do not set the optional StorageResolution parameter, then CloudWatch will default to storing the metrics at 1-minute resolution.
When you publish a high-resolution metric, CloudWatch stores it with a resolution of 1 second, and you can read and retrieve it with a period of 1 second, 5 seconds, 10 seconds, 30 seconds, or any multiple of 60 seconds.
Custom metrics follow the same retention schedule listed above.