For those of you who have not spent more than 20 hours playing Shogun 2 since Christmas:
The Sengoku Jindai, or warring states period lasted for most of the 16th century in Japan. It was characterized by near constant warfare between the rival feudal lords (Daimyo) with all seeking to expand their lands, and a few striving to become the new Shogun. At various points in the conflict different clans were ascendant, with the Oda clan under Oda Nobunaga and the Toyotomi clan under Toyotomi Hideyoshi coming the closest to seizing power in a conflict ultimately won by the Tokugawa clan under Tokugawa Ieyesu.
The conflict involved not just feudal warfare, there was a strong religious element present. On the main island (Honshu) there was a significant uprising of peasants and master-less samurai who followed the radical Ikkō-ikki sect of Buddhism, while on Kyushu and other regions Catholicism made significant inroads. Ultimately religious unification followed political unification: The Ikkō-ikki were crushed militarily, while Christianity forbidden and critically undermined by the complete seclusion of Japan that followed.
So, what is the shortest and longest plausible "Age of Warring States" that Japan could go through during the 16th century? Was there ever a possibility of a non-Shinto dominated Japan emerging? No POD earlier than 1467 or later than 1600.