22 manga chapters, anime unknown
In the manga, Lala uses her tail beam and becomes a child in Chapter 44, "Power and Power ~Protector and Protected~".
It was actually pretty hard to track down when she gets her sexy adult body back, because her child body bears an incredible likeness to Momo, Nana, and Yami in certain scenes, especially when they're naked, which they frequently are, because it's To-Love-Ru. This made it necessary to read a lot of the context, which slowed down a casual scan. But Lala's growth back to her usual body is the main plot of Chapter 66, "Delight ~Growth of Body and Heart~". So it takes 22 chapters.
There's no clear sense of how much time passes in-universe between these two chapters, but once Lala starts growing, it takes her about a day to grow back to her original body, as stated by Peke at the beginning of Chapter 66.
I haven't watched the anime of Darkness, but according to Wikipedia, Lala is still in her child form as of the third episode of the most recent series, the 2016 OVA To Love-Ru Darkness 2nd, so it's unclear when the anime will have her regain her adult body.