16 Weeks. That's what the majors and the bigger distributors are doing.
First four weeks: Barcode assignment, Onesheet set-up, Secure distribution
Second four weeks: Solicitation, mailouts to retailers and wholesalers, pre-promotions, book print advertising.
Third four weeks: Finalize orders, start manufacturing, launch street promotion campaign, launch internet campaign, book visual media advertising (video commercials), launch PR campaign, launch radio campagin.
Last four weeks up to street date: Heavy street promotions, additional mailout of onesheets and teasers, retail fax blitz, radio interviews, promo tours.
Thats just a basic suggested breakdown. Good luck.