Those "six months" is something called "man hours" and it is an industry standard for process of car modeling. The same "six months" timeframe is used by almost all modeler companies that are working on the high-detailed modeling, from those working for the Forza series (Rabcat, GlasssEgg, Dhruva..) to modelers of pCARS.
Logically, modeling cars for games such as iRacing, RaceRoom, rFactor, etc. takes much less time because those cars are very low on details, mesh complexion and shader/textures implementation.
Real time to render a vehicle for GT/Forza/pCARS (exterior and chassis in parts) is around 4 weeks, while interior can take up to 2-4 weeks more, depending on complexity.