Baaaaaad idea!
I once considered nags. As I test, I used the nag-version of my software as my everyday comic-reader. The nag happened at app-exit based on the day of the month (basic n MOD x == 0 ? thing iirc). Well, at first it just irritated, then it annoyed. Then I rebuilt it without the nags for myself. Consequently, I've actually disabled the download (prior to disabling my whole site for renovations, lol). Long and short: do not.
Honestly, I think nags are a pest and there are better ways. If you want people to spread the work about your app, then have a "Like this page: [your product's Facebook page or whatever] and get this cool, awesome bonus stuff". Surprisingly, people like free things and they like bonus things so the math works out. You won't believe the things I've seen people like on Facebook (when I still used Facebook - very briefly).
Quote: "I wish FB would stop being integrated into everything."
Yeah, I agree. It drives me insane. One would think FB was trying to take over the world.
Btw if you "Like" my post you could win a year's supply of potato chips.
Silence is golden, duct tape is silver