How long would a nuclear winter last?

uming that 36,000 megatons of bombs [about 79,000 bombs] went off [just over the world peak megatonnage [in 1974]], they were all in urban "high fire risk" areas, with a great deal of flammable material [wood etc.], as well as all of the world's oil wells being burned [adding more crap into the atmosphere], and that some of the bombs went off over national parks [ume that ALL of the WORLD'S forests are COMPLETELY on fire and will COMPLETELY burn down, thus adding even more soot into the atmosphere], and all of the dust and soot stayed in the atmosphere, how long would it take before most areas of the U.S could be farmed normally [i.e- it was warm and sunny enough for non-greenhouse farming in the U.S breadbasket [midwest], and fruitbasket [san juaquin valley]? How long would the soot and dust stay in the atmosphere?