Thanks for reaching out to us via AWS re:Post.
AWS storage auto-scaling for RDS gets triggered based off the below conditions->
With storage autoscaling enabled, when Amazon RDS detects that you are running out of free database space it automatically scales up your storage. Amazon RDS starts a storage modification for an autoscaling-enabled DB instance when these factors apply:
Free available space is less than 10 percent of the allocated storage. The low-storage condition lasts at least five minutes. At least six hours have passed since the last storage modification, or storage optimization has completed on the instance, whichever is longer.
The additional storage is in increments of whichever of the following is greater:
5 GiB 10 percent of currently allocated storage Storage growth prediction for 7 hours based on the FreeStorageSpace metrics change in the past hour. For more information on metrics, see Monitoring with Amazon CloudWatch
 Working with storage for Amazon RDS DB instances - Managing capacity automatically with Amazon RDS storage autoscaling - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PIOPS.StorageTypes.html#USER_PIOPS.Autoscaling
Auto-scaling does not exclusively wait for the maintenance window.
Hope I was able to shed some light on the matter for you - Have a great rest of the day!
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