I think it depends on what your going for. There are some great articles on watchtime.com about the development of the Tag V4 and that took over 7 years, but it is a pretty wild design. I believe the Breitling B1 chronograph was about 5 years. So my guess is 3-5 years is a reasonable estimate. Now if your doing all the tooling then that could be a whole other time table all together.