Since this post has been going on for nine years now, it's unlikely that the originator of the question (and the early responders) still care what we're saying, but one can find a definitive answer in what I have always considered to be the best reference source for writing: "The New York Public Library Writer's Guide to Style and Usage." (Sorry for the incorrect use of quotation marks, but this site won't support italicization or underscoring.) I quote:

Inanimate Possessives. Many writers claim that because inanimate objects cannot show ownership, the possessive should not be used. But others disagree, arguing that the alternative results in unnecessary wordiness. For example, some writers and editors would change "the train station's platform" to "the platform of the train station," although most today would not.

In phrases of personification of time and money, however, the possessive has always been acceptable.

this evening's storm
dawn's early light
5 days' leave
3 years' salary
6 dollars' worth
Seven Years' War

The possessive should not be used when no possessive relationship exists.

3 years ago
6 years later

[End of quotation]

This approach is echoed by the "United States Government Printing Office Style Manual," as follows:

8.14. The possessive case is often used in lieu of an objective phrase even though ownership is not involved.

1 day's labor (labor for 1 day)
2 hours' traveltime
a stone's throw
2 weeks' pay
5 or 10 billion dollars' worth
for charity's sake
for pity's sake

[End of quotation]

Finally, from "The Elements of Grammar for Writers," is this (at a more basic level):

How can you tell whether a word is possessive or plural? A possessive acts like an adjective, whereas a plural is a noun (nominal). That is, the possessive form of a noun will nearly always have another noun after it. Thus, the following reasoning is used in punctuating the phrase "two months' vacation":

1. Because there is a noun after the word "months," it should be possessive.
2. Now, because there is more than one month involved (two), you have a word that is both plural and possessive.
3. So you write the complete plural, "months."
4. Then you add an apostrophe: "two months' vacation."
5. You add no "s" after the apostrophe because you would not speak a second "s" sound out loud.

[End of quotation]

I won't bore you with additional references, but they all are in agreement: "two weeks notice" or "two week notice" is incorrect; "two weeks' notice" is correct. I now give you but a moment's notice that my comment is ending.