1 Month? I don't think so. Why?
Because Its stuck between The Witcher 3 and Batman AK + The fact that for some reason Nintendo only users are still getting grasp on how online gaming works and may not be interested in a game that relies so much on it.
With that being said, if this game had actually the exact same set up BUT it was called Mario Paintball (with Nintendo Characters) it will sell 5 million copies in 1 year easily. The attachment ratio for Mario games is just ridiculous
"The Witcher" and "Batman" - coming to Wii U??
Both titles are completly irrelevant for the Wii U owner. And they even don't compete with Splatoon as Splatoon defines a new genre within the shooter genre.
If you look closely you'll find that different Nintendo communities like the Zelda and Smash one are hyped about it.
Splatoon doesn't need to be a Mario title. What a luck it isn't. Its a new unique world of its own and it simply works this way.
And Splatoon will not only sell 1 Mio. EASILY within one month but it will have "legs" and get to an attach rate like Smash and Mario Kart.
There is a reason for that: Nintendo release politics. Splatoon will be "fresh" for the next months to come and will be over and over again in media.
The biggest splash will make the team building functionality. In August we will see sales numbers skyrocket again.
And if Nintendo gets its act together it might go on with Splatoon entering the eSports arena.