What is the minimum storage duration billed for an object in the Coldline storage class?

Why this is the answer

Coldline carries a 90-day minimum storage duration: an object deleted or replaced before then is still billed for the remainder of those 90 days. The 30-day figure is Nearline's minimum and 365 days is Archive's, so both belong to neighbouring classes. Standard is the only class with no minimum duration. This is why a lifecycle rule that demotes data too eagerly can cost more rather than less.