When words showing measurement, value, distance, amount are used with an s on the end and they are in front of a noun, use an apostrophe s for singular and just an apostrophe for plural.
...one week's break from this...
...one day's duration...
...two weeks' vacation...
...several hundred dollars' worth...
When using the apostrophe in such a construction, remember that there must be a NOUN that the adjective is modifying.
...six days long...
...three months pregnant...
Good news!! The word pairs book will be out in just a couple weeks. Check www.margieholdscourt.com for the news!
Happy punctuating!
Margie