Ender is said to have started using the video room for tactics after Dragon Army's seventh battle.
At the end of the week Dragon Army had fought seven battles in seven days. The score stood 7 wins and 0 losses. Ender had never had more losses than in the battle with Phoenix Army, and in two battles he had suffered not one soldier frozen or disabled. No one believed anymore that it was a fluke that put him first in the standings. He had beaten top armies by unheard-of margins. It was no longer possible for the other commanders to ignore him. A few of them sat with him at every meal, carefully trying to learn from him how he had defeated his most recent opponents. He told them freely, confident that few of them would know how to train their soldiers and their toon leaders to duplicate what his could do. And while Ender talked with a few commanders, much larger groups gathered around the opponents Ender had defeated, trying to find out how Ender might be beaten.
[two more paragraphs of Ender musing to himself]
Ender was teaching them all about null gravity tactics. But where could Ender go to learn new things?
He began to use the video room, filled visit propaganda vids about Mazer Rackham and other great commanders of the forces of humanity in the First and Second Invasion. Ender stopped the general practice an hour early, and allowed his toon leaders to conduct their own practice in his absence. Usually they staged skirmishes, toon against toon. Ender stayed long enough to see that things were going well, then left to watch the old battles.
But then when Graff and Anderson ask him about it on that day, they act as if he had been doing it for a while.
It was during the last hour of practice on the seventh day, only a few hours after Ender's army had won its seventh battle, that Major Anderson himself came into the video room. He handed a slip of paper to one of the commanders sitting there, and then spoke to Ender. "Colonel Graff wishes to see you in his office immediately."
Ender got up and followed Anderson through the corridors. Anderson palmed the locks that kept students out of the officers' quarters; finally they came to where Graff had taken root on a swivel chair bolted to the steel floor. His belly spilled over both armrests now, even when he sat upright. Ender tried to remember. Graff hadn't seemed particularly fat at when Ender first met him, only four years ago. Time and tension were not being kind to the administrator of the Battle School.
"Seven days since your first battle, Ender," said Graff.
Ender did not reply.
"And you've won seven battles, once a day."
Graff and Anderson exchanged glances. "Ender, why are you studying the videos of the bugger wars?"
"To learn strategy, of course."
"Those videos were created for propaganda purposes. All our strategies have been edited out."
So then when did Ender start using the video room?