Mobile gaming doesn’t often get a blockbuster release, but it has had two so far in 2016 — although the latest has already easily eclipsed everything before it.

Pokémon Go is the fastest mobile game ever to reach No. 1 in terms of revenues on iOS and Android, according to app-market tracking firm App Annie. It reached that position on the charts in the less than a day on iOS and just four days on Android. The previous record holder, developer Supercell’s Clash Royale strategy battler, needed two days on iOS and seven days on Google Play to reach No. 1. Supercell launched that app to massive hype and expectations from its millions of fans still playing long-time top-grossing No. 1 Clash of Clans, but Nintendo appeals to a wider audience.

“Even though Pokémon Go is only officially open in three countries, the game is generating well over $1 million of net revenue for Niantic Labs,” App Annie communications boss Fabien Pierre-Nicolas said. “I can easily envision a run-rate of over $1 billion per year with less server issues, a worldwide presence, and more social and player-vs.-player features.”

If Pokémon Go does generate $1 billion annually, this’ll mean it maintained at least a top-five placement on the U.S. or Japanese app stores (it’s not even out in Japan yet) for 12 straight months. To do that, it’ll have to keep players engaged and spending money. That’s not easy, and the game may struggle to keep players around after this early honeymoon period.

“The Pokémon brand ensures tens of millions of downloads regardless of the game concept,” Newzoo founder and analyst Peter Warman told GamesBeat. “It is one of the biggest gaming and entertainment franchises on the planet.”

Warman, however, is surprised at the amount of money the game is bringing in, and he doesn’t think that this is permanent.

“What has been a surprise for Pokémon Go is the several millions of dollars it’s managed to generate in the first few days alone,” he said. “This is obviously great news for Nintendo as seen by its recent $10 billion market cap jump. My one worry with the game is its longevity as the freshness of the concept wears off and is copied by competitors.”

But even if the concept does attract clones, those apps won’t feature real Pokémon. And app developers have a lot to learn if they think I’m gonna go walk in 95-degree heat for some generic Pikachu knockoff.