You can find pretty much all you should need to know about scaling to your requirements on this web page:
It includes a tutorial for using the auto scaling as well as writing your Lambda function to scale based on time of day.
There are two types of fleets in AppStream 2.0, Always-On and On-Demand.
With Always-On fleets, all instances that are running are charged the applicable running instance fee, based on the instance type and size, even when users aren't connected.
With On-Demand fleets, instances are charged the applicable running instance fee, based on the instance type and size, only when the instances are used for streaming sessions. Instances in On-Demand fleets that are not being used for streaming sessions are charged a small hourly stopped instance fee that is the same for all instance types and sizes.
Another cost you will want to consider is User RDS SAL fees. For more information on pricing, please see: https://aws.amazon.com/appstream2/pricing/
So controlling the costs by scaling your fleet can be done a few ways. Time based scaling is able to be accomplished via the AWS CLI, Link: https://docs.aws.amazon.com/appstream2/latest/developerguide/autoscaling.html#autoscaling-cli
This can be used with other scaling methods such as Available Capacity or Capacity Utilization to ensure that the number of available instances stays inline with the number of active users you have throughout the day.
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