Please know that Mario RPG is a SNES game on Wii Virtual Console, and not a 3DS title, so 3DS has nothing to do with it (except maybe that's where your coins came from?). I just didn't want you to get it thinking it was a different title. You may want to check the list, because I'm not sure if they ever offer the same titles on Club Ninty, so this may be the only chance to trade coins for this title for a while. The code I got from Amazon when World of Goo came out (a few years back) was good when I redeemed it recently, so you won't have to worry about expiration if you plan to use within a few years.
OK but I don't own a Wii or WiiU currently. If I redeem the coins for Mario RPG how long is the download code good for and can I even redeem the coins for that if I don't own a 3DS?
I may just save the coins and see if a different game for 3DS shows up. How often is the coin reward list updated?
Also, this title says it requires the classic controller, so the Wiimote won't work on Wii or Wii U (unless someone can confirm otherwise, contrary to the website).