AWS CloudWatch metric for data transfer into an S3 bucket per min
There are no metrics available for rate of data in/out of Amazon S3.
You could create an AWS Lambda function, triggered on the creation of new objects in the bucket.
The function could record details of new objects created, such as their size and the time. This information could be stored in a database or even in a custom metric in Amazon CloudWatch. You could then use that data to calculate a "new GB per hour" type of metric for files being created in the S3 bucket.
If storing in CloudWatch, use a SUM() aggregate for adding the sizes over the given period of time.