Starting today, Amazon DocumentDB instances are billed in one-second increments. With on-demand pricing and no up-front commitment required, Amazon DocumentDB’s per-second billing provides additional granularity, so you only pay for capacity you use. Partial instance hours are billed in one-second increments, with a 10-minute minimum charge following a billable status change such as creating, modifying, or deleting an instance. Per-second billing applies to all existing and new instances in all regions in which Amazon DocumentDB is available. Please visit the Amazon DocumentDB pricing page for more information.
from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/05/amazon-documentdb-now-supports-per-second-billing/