Gold Points in the eShop expire in 12 months after receiving them. If you are saving Gold Points for a later purchase check your expiration status before it gets deleted.
You can check the expiration status of your points at https://my.nintendo.com/point/expiration
Gold Points is received when someone purchases through the eShop (5% of the amount of actual money you pay) and from physical copies. With physical copies, select the game, press + and go to “My Nintendo Rewards Program”: You get 1% of what the digital version of the same game costs in the eShop, one time per game card, and the points must be claimed within 1 (NA) or 2 years (EU) of the game’s official release date.
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According to Nintendo Customer Support
Do My Nintendo Points Expire?
Yes. Different types of points are valid for different lengths of time:
- Gold Points are valid until the last day of the month 12 months after you earn them. (For example, points earned on April 15, 2017, would expire on April 30, 2018.)
- Platinum Points (all types) are valid until the last day of the month 6 months after you earn them. (For example, points earned on April 15, 2018, would expire on October 31, 2018.)