How long does AMI pending take?
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.
Why does AMI creation take so long?
The most common cause for slow creation of AMIs or snapshots is the amount of dirty data that must be copied to Amazon S3. This dirty data is measured by the number of blocks. The following factors might result in a large number of dirty blocks: Size of the EBS volume.