John Pulman was the king of the 1960s, when the world championship was played on a challenge basis. However, when the tournament reverted to a knockout formula in 1969, he did not prosper.  Ray Reardon became the dominant force in the 1970s, winning six titles, with John Spencer (snooker player) winning three. Steve Davis first world title in 1981 made him only the 11th world champion since 1927, including the winner of the boycotted 1952 title, Horace Lindrum. Stephen Hendry became the 14th in 1990 and dominated through the 1990s. Reardon won six (1970, 1973–1976 and 1978), Davis also six (1981, 1983, 1984 and 1987–1989) and Hendry seven (1990, 1992–1996 and 1999). Ronnie OSullivan is the closest to dominance in the modern era, having won the title on five occasions in the 21st century (2001, 2004, 2008, 2012 and 2013). Mark Williams (snooker player) has won three times (2000, 2003, and 2018) and John Higgins (snooker player) four times (1998, 2007, 2009, 2011) but since the beginning of the century, there has not been a dominant force like in previous decades, and the modern era has seen many players playing to a similar standard, instead of one player raising the bar. Davis, for example, won more ranking tournaments than the rest of the top 64 players put together by 1985. By retaining his title in 2013, OSullivan became the first player to successfully defend the world championship since 1996 when Hendry won the sixth of his seven titles, his fifth in a row, and then later by Mark Selby in 2017.
Question: How many years passed btween OSullivan's first title win and his last one?
Answer: 12