(Threads now has a fixed 15 minutes delay before they federate a post)

Generally, when describing a thing, the unit is written as singular. "15 minute delay", "10 foot pole", "5 gallon bucket". When referring the unit itself though, it would be plural: "a delay of 15 minutes", "the pole is 10 feet long", "this bucket holds 5 gallons". I'm sure there's a more precise way to say this, but hopefully it helps.

The delay is singular. So one delay for fifteen minutes is a fifteen minute delay.