How do I get data storage used on my AWS RDS?

Use:

aws cloudwatch get-metric-statistics  --start-time $(TZ=PST+1 date +%FT%T) --end-time $(TZ=PST date +%FT%T) --period 3600 --namespace AWS/RDS --statistics Maximum --dimensions Name=DBInstanceIdentifier,Value=${yourRDSname} --metric-name FreeStorageSpace

How I got --dimensions Name:

aws rds describe-db-instances --output json | less

How I got metric-name:

http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/rds-metricscollected.html

Have a look at the FreeStorageSpace metric.

It can be found in Cloudwatch, under RDS metrics.

From the RDS Instances console, you can select an instance and click "Show Monitoring". The "Freeable Space" chart is the same information. Click the chart for more details.