A little over four-and-a-half years. TSMC announced the shipment of the company's one-millionth 90-nanometer process 300-mm wafer, which took place 53 months after process launch.
Obviously process generations overlap. The 90-nm generation was in development for years before the first wafers shipped, and will continue to produce products for years into the future. And obviously companies with less capacity than TSMC will take longer to reach the million-wafer milestone.
Though most industry coverage has moved on to the next thing by the time the first wafers from a new generation ship, it's good to remember that the process's economic impact is just beginning at that point.