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.

[–] maniclucky@lemmy.world 3 points 1 month ago

In the first case, the subject (object? I always get them confused) is delay (which is singular), and the adjective is "15 minute".

In the second, the thing is "minutes" (plural) modified by "15".