The 777 program reached this milestone after 11 years in service, faster than any other twin-aisle airplane program in history.

If the 787 really only takes 3 days to assemble it will achieve this milestone much quicker. We estimate the 787 could reach this milestone between 4.5 to 5 years, half the time of the 777. But Boeing has to sell 600 first.

By comparison with other aircraft deliveries:
A330(1988) - 2004 - 16 Years
A300(1974) - 1989 - 15 Years
B747 (1969) - 1984 - 15 Years
B767 (1982) - 1996 - 14 Years
B777 (1995) - 2006 - 11 Years

More here…