In episode 34 Near asks Mogi how long you can manipulate a victim before their death. He replies 23 days because the Yotsuba incident proved this? When in the data from the Yotsuba incident was this proved?
It was revealed because of Misa in chapter 48. Higuchi promised to stop killing criminals, so he could marry Misa, the second killer and he did. But because he had written down lots of criminals already, the deaths kept on continuing for 23 days until they stopped. From this they could deduce the killings could only be manipulated until 23 days in the future.
The death note that was dropped by Ryuk contained all the known rules including the 23 days rule. After Light made the Shinigamis switch the books, Yotsuba got the one with the rules that later came to the investigators.
The "fact" that the deaths stopped after 23 days is wrong btw. They explicitly stopped immediately because he never wrote a date of death for criminals.