In the North American My Nintendo, new offers are usually added around the first of each month. A good indicator of when this occurs is by checking the expiration dates of the oldest offers. I can't really comment on an exact time.
Once in awhile the PC wallpaper section seems to be updated on its own accord.
Currently playing: Mario & Luigi: Bowser's Inside Story + Bowser Jr's Journey, Ys VIII: Lacrimosa of Dana (Switch)
It's as another poster said usually at the beginning of the month and if you have Silver coins unused those expire but not the Gold coins. So use the Silver coins when possible.
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Topic: Nintendo Rewards (reset)
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