My Little Pony XLVIII: I say Ol' Chap, how about those Equestrians?

[Anarion]

I was wondering primarily which of the two sounds more adequate, but also if there are better suggestions I will steal use them.

Well, between the two, "10 days" is more vague about what happens, but more clear about the time period. If I know it's 10 days during which cool stuff happens, I have a good sense of what character growth will look like, how many different locales the story will use, and the emotional tenor of the story.

If the title is "The Sun is Heavy" I get a better understanding about what happens in the story (I haven't read your fic, but I assume from that title that it's about Celestia in some respect and that she gets tired or gives up her work somehow). So, one is informative about the plot narrative, and the other is informative about the timeframe and setting.