A three hours' journey a four weeks' course

Anna232 said:

As "three hours' journey" and "four weeks' English course" are wrong while "a three-hour journey" and "a four-week English course are right. I am trying to find out why in some cases 's or a - are both correct (a five-minute walk, five minutes' walk) but in some cases only the hypen is used ("a three-hour journey" and "a four-week English course). Does it depend on each word? Is there a particular rule?

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emsr2d2 said:

I'm sure someone will come along and prove me wrong, but I would say that we use "[no article] number, apostrophe, s" only with periods of time together with certain methods of travel.

It's a one-minute bike ride.
It's one minutes' bike ride.
It's a twenty-minute jog from there.
It's twenty minutes' jog from there.
It's a five-minute walk.
It's five minutes' walk.
It's a four-hour drive from here.
It's four hours' drive from here.

It's a ten-year plan.
It's ten years' plan.
It's a three-hour journey.
It's three hours' journey.
It's a twelve-week French course.
It's twelve weeks' French course.

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Anna232 said:

Thank you so much. That's exactly what I was trying to find out no full stop here but when both are used, there is no difference in meaning, is there? For example, "a two-week holiday" and "two weeks' holiday" no comma here mean the same, don't they?

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emsr2d2 said:

Note my changes above. I always discourage learners from trying to start sentences with "But". It can be done but it's usually better to use it as a conjunction.
I've crossed out "when both are used". That suggests that you want to use the two phrases together in one sentence. That wouldn't make sense. You would use one or the other.
Remember to mark out words/phrases you're asking us to consider in some way. I've put them in quotation marks above.
Whenever you ask us if two things mean the same, give us complete sentences to consider.

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Anna232 said:

I mean if I use either, isn't the meaning the same?

1. I am going to have two weeks' holiday.
2. I am going to have a two-week holiday.

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