If you're looking for a serious answer, the problem is that it's very difficult to pick a specific beginning point for when a design begins and when it's "finished."

The car fanatic magazines fairly often "scoop" the competition with pictures (usually showing nothing meaningful) of new models as much as a year before they appear up on the showroom floor. If you count recall actions and "factory authorized repairs" the design often isn't "finished" until about 3 years after they start selling them.

You may be able to find a copy of the SAE Journal (formerly the Society of Automotive Engineers, but more recently just "SAE International") occasionally on a news stand, which frequently includes "concept car" designs with predicted "future delivery" claims. Most of these are never actually are produced for sale, but you can get a rough idea of how far ahead the stylists are thinking.

A new engine or new suspension geometry that incorporates a significant change may take 3 to 5 years in design and testing, after it has been selected for use in a specific new model - assuming that all of the "concepts" have been proven before you start. Once the design is "final," the engine or axle supplier will need at least a year to solve the production details to be able to get one to the assembly line every few minutes.

If you're just doing a "mix and match" using components that are already "in the pipeline," you might be able to turn out a "new model" with only "cosmetic" changes within as little as 1 or 2 years from "concept."

There are a certain few "radical new concepts" that get announced as "coming out next year" on an annual basis - that have been announced nearly every year for more than the last 30 - and still they haven't shown up in the market because they don't work - - - yet(?). So how long did the design take when/if they do final get there?

John