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a fixed 15-minute* delay
I'm not a native speaker.. interesting, why would that be phrased that way? It's still multiple minutes?
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.
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".
The delay is singular. So one delay for fifteen minutes is a fifteen minute delay.
Not sure what that has to do with anything. Two delays are plural but two delays for fifteen minutes each would still be "two fifteen minute delays".