According to https://en.wikipedia.org/wiki/Intel_8086: "The architecture was defined by Stephen P. Morse with some help and assistance by Bruce Ravenel (the architect of the 8087) in refining the final revisions. Logic designer Jim McKevitt and John Bayliss were the lead engineers of the hardware-level development team and Bill Pohlman the manager for the project." I expect the microcode was developed in tandem with the rest of the chip, so probably took about 2 years.