Indefinite article

[sergeyrais]

Is it right to put an indefinite article before a phrase with a noun modified by a genitive which includes a plural noun?

e.g. We had a two weeks' holiday in Greece.
There was a thirty minutes' lesson yesterday.

Should the lack of the indefinite articles in the above sentences be considered as a grammar mistake?

[emsr2d2]

Is it right to put an indefinite article before a phrase with a noun modified by a genitive which includes a plural noun?

e.g. We had a two weeks' holiday in Greece.
There was a thirty minutes' lesson yesterday.

Should the lack of the indefinite articles in the above sentences be considered as a grammar mistake?

Neither of those sentences should contain the indefinite article.

We had a two-week holiday in Greece.
We had two weeks' holiday in Greece.

There was a thirty-minute lesson yesterday.
(This cannot be converted in the same way as the first sentence).

[sergeyrais]

Dear emsr2d2,
Can't we use an indefinite article referring it to the head-noun modified by a plural noun in the genitive case considering the latter as a descriptive attribute? So, what's wrong with the following word combinations:
a soldiers' canteen, a girls' school, a three miles' walk, a fifteen minutes' break, etc. ?