I was first playing with dynamo when the autoscaling feature was released... it wasn't great at the time (neither was manually scaling). It's nice to see the feature is working better now.
Normally, you just write to a data store until it's done. The faster the disks the less time it takes.
Now, writes can "fail" so you need to wait/retry or put it back on a queue to reprocess when you have write units available (both of those and the associated logging will cost you $ on AWS).