If you look at the timeline of the amd64 architecture: http://en.wikipedia.org/wiki/X86-64#History_of_AMD64

Then it only took 8 years to make a 64-bit port from the date of the first available amd64 machine. If you take into account the date of the first full spec released to the public, it's almost 11 years.

Now if only complex software like the Linux Kernel could be ported in shorter time....