The title "top percent" reflects that a user is in an artist's top listening base, essentially meaning they listen to their music more than the average listener.
The way that Spotify calculates if you're in an artist's top percent hasn't ever really been fully disclosed, but it's safe to assume that the app takes into account a variety of factors when judging, including both total time spent listening and the number of songs played.
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The data is collected from Jan.1 to "a few weeks before launch," per the Spotify FAQ page. Spotify then calculates and prepares the yearly wrapped for a late November or early December delivery.
If you're eager to get into an artist's top percent next year, the best bet is to start listening as much as you can right on New Year's Day.