PokéBase - Pokémon Q&A
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It does not appear to expire. It looks to be permanent, at least until Switch's online funcionality is retired.

Every Poké Ball Plus contains your very own Mythical Pokémon Mew, a special Pokémon that cannot be obtained by normal gameplay in any of the main Pokémon series games.

https://pokeballplus.nintendo.com/

Just remember although there doesn't seems to be a deadline, you can only redeem Mew once.

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