It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.
Naruto could develop by learning to adapt his ninja skills to a more modern and zombie - filled setting. He might become more cautious as he realizes the different threats here compared to his ninja world. Rika could develop from being a normal schoolgirl to a more confident and brave fighter. She could learn from Naruto's fearless attitude and start using her own wits in more daring ways.