Board Thread:Naruto Discussions/@comment-26135923-20150707130017/@comment-26275777-20150711130021

By "every generation" I obviously don't mean "once every 25 years". Even if it was every sixty years or so, that's 16 times fresh pure chakra would be entering the Senju/Uchiha lineages.

And Madara was the exception, not the rule - he and Hashirama were the first to live in peacetime and he cheated with the Gedou Mazou. Hashirama himself said it: average life expectancy was more like thirty years in his time.