Spars with Shisui for 3 hours

“Hah!” Expelling a breath full of ambition, Shisui looked at Itachi and
smiled. “Yeah, training with you forces me to really put my back into it.”
Itachi looked at his cheerful friend and felt refreshed in both mind and body, as he stood there drenched in sweat.

They had been sparring seriously for three hours or so, as a way for Itachi
to vent his anger with his assignments. They had already gone fifteen rounds,
with breaks of about three minutes after each one, resulting in six wins for
Itachi and nine for Shisui.

The only rule was no sharingan; everything else was fair game. Sharingan
was off limits because the visual technique used up a large amount of chakra,
and they wanted to spar for as long as possible.
The shuriken they flung at each other clashed in the air, and flew off in
some unexpected direction. Neither Itachi nor Shisui watched to see where
they went. They were already leaping at each other, and closing the distance
between themselves.

“Chi!”
“Hah!”
Their fighting cries blended together.
Their bodies collided in midair, tangled together, and crashed to the
ground.
Shisui got to his feet and in position first, and launched a kick at Itachi,
who was propped up on one knee. Itachi immediately thrust his right arm out
to block it. His field of view shaking with the impact of the kick, he saw his
friend quickly weaving signs.

Book of Bright Light Chapter 2 Part 4