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.
Punctuation Frustration
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.