When will the future Buddha Maitreya replace the Tathagata and become the Lord Buddha?

We cannot say "replace". Nowadays, it is propagated on the Internet that the Dharma of Sakyamuni Buddha cannot save sentient beings. Maitreya must re-transmit the Dharma. There is no such thing! Buddhas and Buddhas have great enlightenment. How can this Buddha's Dharma be able to save all living beings? Which Buddha's Dharma cannot save all living beings? This is a rumor.

So? Why did Shakyamuni Buddha pass away? It’s not that the Dharma can’t save sentient beings! But the Dharma connection is gone! No one will listen to him anymore, and his law will be destroyed. The Buddha's destiny lasts for twelve thousand years. Now three thousand years have passed, and it can still benefit sentient beings for nine thousand years! After nine thousand years, Buddhism will be extinct! We have to wait until 5.67 billion years later, when Maitreya Bodhisattva becomes a Buddha under the Longhua tree, then we can hear the Dharma again.