At which point in time did Ender start using the video room to study Bugger tactics?

Question

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."

Ender nodded.

[more dialog]

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."

"I know."

So then when did Ender start using the video room?

Answer 1

According to "Ender's Shadow", he started well before the 7th battle - he started months before he was even given Dragon Army because Bean commented on it while he was still studying Ender from afar, before he even was given a task to put together the army's roster by Graff and Anderson:

But even as he tracked Wiggin -- still from a distance -- what Bean could not let himself do was what the younger kids did, what Wiggin's disciples did. He could not call him Ender. Calling him by his last name kept him at a distance. A microscope's distance, anyway.

What did Wiggin study when he read on his own? Not the books of military history and strategy that Bean had blown through in a rush and was now rereading methodically, applying everything to both space combat and modem warfare on Earth. Wiggin did his share of reading, too, but when he went into the library he was just as likely to look at combat vids, and the ones he watched most often were of Bugger ships. Those and the clips of Mazer Rackham's strike force in the heroic battle that broke the back of the Second Invasion.

Bean watched them too, though not over and over again -- once he saw them, he remembered them perfectly and could replay them in his mind, with enough detail that he could notice things later that he hadn't realized at first. Was Wiggin seeing something new each time he went back to these vids? Or was he looking for something that he hadn't yet found? Is he trying to understand the way the Buggers think? Why doesn't he realize that the library here simply doesn't have enough of the vids to make it useful? It's all propaganda stuff here. They withheld all the terrible scenes of dead guys, of fighting and killing hand to hand when ships were breached and boarded. They didn't have vids of defeats, where the Buggers blew the human ships out of the sky. All they had here was ships moving around in space, a few minutes of preparation for combat.
War in space? So exciting in the made-up stories, so boring in reality. Occasionally something would light up, mostly it was just dark.
And, of course, the obligatory moment of Mazer Rackham's victory.
What could Wiggin possibly hope to learn?

Note that this was months before Dragon Army:

And so passed the weeks, the months. Bean did all his regular classwork. He attended the regular battleroom classes with Dimak teaching them how to move and shoot, the basic skills. On his own he completed all the enrichment courses you could take at your own desk, certifying in everything. He studied military history, philosophy, strategy. He read ethics, religion, biology. He kept track of every student in the school, from the newly arrived launchies to the students about to graduate. When he saw them in the halls, he knew more about them than they knew about themselves. He knew their nation of origin. He knew how much they missed their families and how important their native country or ethnic or religious group was to them. He knew how valuable they might be to a nationalist or idealist resistance movement.

And he kept reading everything Wiggin read, watching everything Wiggin watched. Hearing about Wiggin from the other kids. Watching Wiggin's standings on the boards. Meeting more of Wiggin's friends, hearing them talk about him. Bean listened to all the things Wiggin was quoted as saying and tried to fit them into some coherent philosophy, some worldview, some attitude, some plan.

...

Soon Bean found himself being advanced out of classes whose work he had already mastered and being plunged into classwork with older and older groups, who looked at him with annoyance at first, but later simply with awe, as he raced past them and was promoted again before they were half done. Had Wiggin been pushed through his classwork at an accelerated rate? Yes, but not quite as fast. Was that because Bean was better? Or because the deadline was getting closer?

For the sense of urgency in teacher evaluations was getting greater. The ordinary students -- as if any child here were ordinary -- were getting briefer and briefer notations. They weren't being ignored, exactly. But the best were being identified and lifted out.

Answer 2

I'm not sure about the premises in your question.

First, it doesn't explicitly state that Ender started watching the videos after the seventh battle. It's just that after the passage of the seventh battle, we see into the difficulties Ender had in keeping ahead of teaching his army. They can learn from him, but where does he learn from - he could have easily started before the apparent time in the book.

Second, even just an afternoon of watching the videos might have cause concern for Graff and Anderson.