The overall development takes 2-5 years, but designers spend only up to a year on the actual design process. The rest of the time is spent trying to make sure the design does not stray too far away from the intent. Robert Cumberford wrote that there were maybe 2,500 automotive designers in the world, with perhaps 1,000 of those working directly for car companies. Automotive design is a type of industrial design, a profession which is not well known in itself. A degree is necessary to get a job.
Most schools require a portfolio as part of the admissions process.
The top schools, make that the only schools, that offer transportation design programs in the U.S. are CCS , Art Center , Cleveland Institute of Art, and Pratt Institute. I've become aware of other programs in Spain, France, Sweden, and of course Italy. Australia has one or two also. Most car designers are graduates of these design programs. However of these graduates, only a small percentage actually get positions in the automotive industry.