Hmm...at first I disagreed, but your update was something I can deffinatly agree with. It took Madara most of his life to awaken the Rinnegan. So that means despite his powerful Uchiha DNA and Hashirama's incredible Senju DNA...it took time to form the Rinnegan....I could see the Rinnegan taking a longer time to form the weaker and weaker the individuals who mix DNA are!
Interesting insight Derp!!
EDIT: Oh and it also reminds me of the Rinne Tensei no Jutsu...the longer an individual, the more time to revive them...except here it meanst the weaker the Senju and Uchiha are, the longer it takes to Rinnegan Up!