word choice - Is "They all had 15 minutes waits" grammatically correct?

I was reading an article that used the phrase "15 minutes waits" and it sounded odd to me. I've always said "15 minute waits". Which is grammatically correct? Or are they both acceptable?

Google seems to indicate that "minute waits" (157k) is much more common than "minutes waits" (11.3k).

It also occurs to me that perhaps the former is only valid when 15-minute is hyphenated. i.e. "They all had 15-minute waits."

EDIT:

To be clear, the context of the original sentence was that every one of them had a 15-minute wait. Is it ever correct to say "They all had 15 minutes waits"?

Answer

When you use a quantity and a unit as an adjective, the unit is singular:

• A 200-pound man...




• A 280-calorie snack...

When the unit is used as a noun, it's plural (unless the quantity is one, of course):

• 200 pounds of man crashed down on me...




• I enjoyed those 280 calories...