@lis: Das Posten von mindestens zwei gleichbedeutenden Anfragen in verschiedenen Foren wird mit LEO-Verbot nicht unter 24 Stunden bestraft. :-)

Hier geht's weiter:
  related discussion:ein zweiwöchiger Aufenthalt

either is correct, but if you say "a two-week stay" you should use a hyphen, because this is then a compound adjective, and it should be made clear that the "two" refers to "week" and not to "stay". It doesn't matter much in your case, but on principle.

Sometimes it does:

"Two-week stays are cheaper than three-weeks stays."

Imagine that without the hyphens!

The version with the genitive plural is less usual, but possible.

Often seen, but in my view incorrect, or sloppy, is "a two weeks stay".

I agree 100% with escoville, but need to clear up one typo to prevent confusion.

An unwanted plural s snuck into this sentence, which I am printing correctly here:
"Two-week stays are cheaper than three-week stays."

Thank you girly girl. A very unwanted typo.

Thanks everyone!

So, in general, can you say that a two-week stay and a 10-day trip and a 5-minute drive do not need "s apostrophe"? Does anybody know a source that shows all the rules to this issue?

I think this page looks nice, but you are the best judge of what you need.

www.grammar-monster.com/lessons/lhyphen.htm

Danke, sehr hilfreich!

@ lis

With the (very necessary) correction, I think my contribution (above) contains all the rules.

Note perhaps also: a plural noun cannot normally be used adjectivally in English, hence a two-week stay, NOT a two-weeks stay. You can say "a two weeks' stay" because that is not adjectival, but a genitive structure. (If the phrase would be misleading otherwise, then you can use a plural noun as an adjective, as in "a centuries-old castle".) Note that nouns normally only used in the plural also become singular when used adjectivally: a scissor-movement, a trouser-press, a goggle-cleaning fluid.