Is it twice as valuable as one week's worth of notice?
That would depend on one's boss, I suppose. My point was that two weeks' notice is a shortening of two weeks' worth of notice. One week's worth, being singular, takes the single-placement apostrophe. Two, three, or ten weeks would take the plural placement. Perhaps my word 'phrase' was improper; possibly, 'meaning' would have been better:
two weeks' notice means two weeks' worth of notice. Am I starting to be clear, yet?