4 to 5 years, but depends on which brand and company. The japanese like Toyota, are very efficient and they develop a new model in approx 4 years.
A general outline:
Year 1- Board/Discussions, management and project planning, budgeting, marketing research. And later in the year, technical specifications, packaging and vehicle pre-development. Tenders to suppliers.
Year 2- Advanced vehicle development, technical integration, product design and development. And a bit of trialing and testing.
Year 3- Vehicle testing and evaluation, refining, quality control and review.
Year 4- Vehicle testing, production planning, pilot manufacturing.
And finally launch.
This process obviously varies from different companies, and it's expected to get shorter with the advancement in simulation technology CAD/CAE etc.