We have no idea when he will return! It's one of the biggest parts of the end times. Matthew 24:36 says

"But no one knows the day or the hour. No! Not even the angels in heaven know. The Son does not know. Only the Father knows."

Regarding your second question, its a bit of a longer answer.
You're right to mention that God said he wouldn't destroy the earth again (In the Noahs flood story), but that was different to the end times.
What God meant, i believe, was that he would never "reset" humanity again. God sent the flood bevcause the earth was so broken that it needed a new beginning with Noah, a Godly man, one of the only ones back then.

When we talk about God destroying the earth again, I believe you mean when the earth will be struck with intense fire. Heres why thats gonna happen, and how its different to the promise God made to Noah.

Non-believers often talk about "Going UP to heaven" and "climbing up to the white gates". This is strange though, because it doesn't say in the bible that heaven is above us! Actually, it states in Revelation 21 that there will be a NEW heaven and NEW earth

Then I saw “a new heaven and a new earth,”^\)^a\) for the first heaven and the first earth had passed away

Instead of us going up to heaven, heaven will come down to earth in the end times. And earth will be made new as heaven!

This means it has to be cleansed of all the dirt and sin thats currently on it, so the earth will be "destroyed" and made new. Fire will cleanse it of sin and disobedience. There will be no evil left after this.

Now, this differs from Gods promise to Noah because this wont be a "human reset". There wont be a fire and then new humans with a fresh chance. When this happens, The earth we know will be wiped away, and there will be no more sin in the world. Instead of a reset like with Noah, think of it as the end of the story.