I often write about the writing problems that I encounter, and this is one that just won't go away. I have so much trouble when it comes to apostrophes and time, I'll usually find a way to re-write the sentence instead. The rule is confusing, and it's one that you'll have to memorize in order to get right.
You can wait for a package to come for months, and you can wait for a month of Sundays. You can even wait a week's time for that package to come to your door. But when you have two weeks' worth of waiting piled up, punctuation gets totally flipping confusing. Yes, I'm about to come up with some better examples of how to do it correctly.
If you've got a week's worth of blog posts waiting for you, you're right. But if you have two weeks' worth of garbage in the can, you're also right (only punctuationally -- go take out the trash!).
When it comes to apostrophes and time, the punctuation follows the same basic rules as other plural possessive words. That means you can have a day's pay. It's possessive, and there is only one day. You know what that means: the apostrophe goes before the s.
But you can also have three weeks' leave. In this case, there is more than one week. Now the apostrophe has to move, because when something is plural possessive it goes behind the s.
How do you know where to put the apostrophe? Look at the measure of time you're using, and decide if it's plural or not. If there is more than one day, week, month, year or decade, it's plural. Remember that, and you'll get it right every time.