context: Metallica was released on August 12, 1991, and was the bands first album to debut at number one on the Billboard 200, selling 598,000 copies in its first week. It was certified platinum in two weeks and spent four consecutive weeks atop the Billboard 200. Logging over 488 weeks on the Billboard 200, it is the third longest charting album in the Nielsen SoundScan era, behind Pink Floyds Dark Side of the Moon and Carole Kings Tapestry (Carole King album). In 2009, it surpassed Shania Twains Come On Over as the best-selling album of the SoundScan era. It became the first album in the SoundScan era to pass 16 million in sales, and with 16.4 million copies sold by 2016, Metallica is the best-selling album in the United States since Nielsen SoundScan tracking began in 1991. Of that sum, 5.8 million were purchased on compact cassette. The album never sold less than 1,000 copies in a week, and moved a weekly average of 5,000 copies in 2016. Metallica was certified RIAA certification by the Recording Industry Association of America (RIAA) in 2012 for shipping sixteen million copies in the US. Metallica sold 31 million copies worldwide on physical media. All five of Metallicas singles, "Enter Sandman", "The Unforgiven", "Nothing Else Matters", "Wherever I May Roam" and "Sad but True" reached the Billboard Hot 100.
question: How many years after Nielsen SoundScan tracking did Metallica become the best selling album in the US?
answer: 25