MMAJunkie posted an article regarding Dustin Jacoby, who will go from his non-UFC pro debut to a UFC card in 11 months.
The quickest ever was Koji Kitao, who made his UFC debut at UFC 9, just 42 days after his non-UFC pro debut. Since 2000, the record belongs to Frank Mir, who did so in 111 days. Since 2010, John Salter just edges out Jacoby, having done so in 290 days.
Of course, this statistic excludes fighters like Matt Mitrione, who had the benefit of “The Ultimate Fighter”.