When did Uchiha Madara actually bite Hashirama Senju? After reading this question, I realized that the time Madara acquired Izanagi, was the same time he got the DNA of Senju Clan from Hashirama. However, after an extensive review on their fight (Episode 626), I can't see where and when Madara bit Hashirama. It said in the wiki that:  

"...To use Izanagi to its full potential, users must also have the genetic traits of the Senju, who are also descended from the Sage.."

Now, the question is when and where did Madara bite Hashirama?
How come Madara activated Izanagi (I assume on its full potential) just right after he acquired the flesh/blood from Senju?
I haven't been keeping up with the anime (too many fillers...), so I don't know whether this manga chapter has been covered or not, or if you are even close to it. This was actually shown in one of the manga chapters quite recently.

If you don't mind spoilers, here are links to a few pages from Naruto Chapter 681:

Madara bites Hashirama

Madara uses Hashirama meat

According to Narutopedia:


  In the past, Uchiha clan members used this technique during an important battle that they had to win at all costs. However, they soon started to abuse the abilities granted by this technique and as such Izanami was created as a way to punish the abusers of Izanagi.


So, I am guessing that the Uchiha must have performed some experiments. Maybe as long as you have the both trait in body you can use Izanagi immediately, and Madara was aware of this. Since Madara clearly must have bit onto the meat, he must have swallowed some blood, thus having the Senju trait in his body. This is an assumption, though.