How does the creation of an AMI on AWS work and why is the creation time unpredictable?
When creating an AMI, you are making a copy of your EBS volumes. This utilizes block storage and will only need to process the diffs of your EBS snapshots. If you changed a lot of data in your EBS or don't have a previous snapshot, it will take longer to create your AMI.

Here is a quote from the documentation at https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/Creating_EBSbacked_WinAMI.html


  Depending on the size of the volumes, it can take several minutes for
  the AMI-creation process to complete (sometimes up to 24 hours). You
  may find it more efficient to create snapshots of your volumes prior
  to creating your AMI. This way, only small, incremental snapshots need
  to be created when the AMI is created, and the process completes more
  quickly (the total time for snapshot creation remains the same).