According to Serebii.net:
In The Games Department
Pokémon GO - Shiny Meltan Event Repeat
It has been confirmed that Shiny Meltan to re-appear in Mystery Boxes in Pokémon GO from April 24th 20:00 UTC to May 5th 20:00 UTC. With this, Mystery Boxes will once again only need 3 days for use.”
GoHub article has yet to come but I’ll edit this post when it exists.