Salam and Hi everyone.

I want to ask, which one is correct between these two sentences?
1# My 3 hours journey was quite boring.
2# My 3-hour journey was quite boring.

and my questions are:
1. Which one is correct? and Why? curious
2. If 2# is correct, since 3 is a plural, is it my 3-hour journey or 3-hours journey?

Thanks for your time!
When a measurement is used as an adjective, the units of measure is singular and the amount+units is hyphenated.
Otherwise, it is plural.

He jumped a three-foot fence.
He walked three miles.
It is three hours until dinner and I'm hungry already.
He went on a three-hour run.

He went on a thousand-mile journey.
The three-hour journey is boring.
My three hours' journey is also correct but far less common.

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 Cool Breeze's reply was promoted to an answer.
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Oh... I see.. Thank you!
And I might add that "a journey of three hours" is also acceptable, but not used very often.

It was helpful.

Thank you.

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First is incorrect because if the word "hour" has s it might be with unobtrusive (three hours') but when there is hyphen it might be without s, consequently second is correct.