When will AWS S3 delete expired objects?

This week I ran into a production issue, because one of my Lambda's choked on the amount of objects in my S3 bucket, which is normally limited, due to a Lifecycle Policy I set on the bucket. Despite the policy, which should have automatically cleaned up any objects older than 24 hours, there were objects as old as a week in my bucket. So what was going on?

Diving into the details of S3's Lifecycle Policy, it quickly became apparent that AWS only guarantees you will not be billed for expired objects. AWS gives no guarantees when expired objects will actually be cleaned. In my experience, this is usually a day within expiring -- except for this week. An inquiry at Amazon AWS Support confirmed this:

Thank you for the provided information, I have checked the LifeCycle Management for the eu-west-1 region and have noted that there has been a recent increase in the number of objects to be expired. As the LifeCycle Manager is asynchronous for all S3 Buckets the region, increases in the number of objects to be expired can introduce longer delays [1].

So please be aware of this, when you're crafting your AWSome solutions!

[1] https://docs.aws.amazon.com/AmazonS3/latest/dev/lifecycle-expire-general-considerations.html