Board Thread:Naruto Discussions/@comment-30881771-20170604165052/@comment-1920550-20170605221730

He died when Obito left the cave to work as his proxy, in fact Madara chose to end his life after turning Obito to his cause by severing the cords that tied him to the statue and fueled him chakra.

He was already near the end of his life even with the support of the Demon Statue. And at the time Kabuto was a nobody that had just been taken in by Orochimaru as a kid (he's actually 5 years younger than Kakashi and Obito). Lastly, given the time span between Oro's demise, Kabuto's "snake ascension" and the war itself, was within a single year, it means that Kabuto got his hand on "some part" of Madara with less than 6 months at best, long after Madara's death.