How long did it take God to create the world?

God created the heavens and the earth in the beginning as recorded in Genesis 1:1 - 2:3. It took God six days to accomplish creation and on the seventh day God rested from all His work.

Genesis 1:1: (KJV)

^1.1In the beginning God created the heaven and the earth.

Verses 1:31 - 2:3

³¹ And God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day.

^2.1Thus the heavens and the earth were finished, and all the host of them.

² And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had made.

³ And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.

Genesis chapter 2 continues to detail mankind and the garden of Eden, with chapter 3 detailing what happened within that garden resulting in mankind being driven out from it.

Exodus 20:11:

¹¹ For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.

Exodus 31:17:

¹⁷ It is a sign between me and the children of Israel for ever: for in six days the LORD made heaven and earth, and on the seventh day he rested, and was refreshed.

By studying the years of those mentioned in the Bible we learn that Adam was created about 4,000 years before Christ.

What caused the earth to "become without form and void"?

The idea (theory) that the earth was beautiful and then it "became" devastated between Genesis 1:1 and 1:2 is not supported in the scriptures.

The record is:

Genesis 1:1 the introduction,
Genesis 1:2 - 2:3 the details (the beginning of verse 2 is a continuation of verse 1 as to the beginning of the earth, but the end of verse 2 tells us what God did first in order to create the heaven and the earth);

...and so altogether between Genesis 1:1 and 2:3 we have God's creation of the heavens and the earth.

God shows it in a step-by-step process - from start to finish. It is like building a house: it starts with a piece of land which is unformed and unfilled, then you put foundation on it, then frame and roof the house, put the things in it, then the people come to live in it. God's purpose came to pass. He did what He wanted to do as recorded in scripture. He created it in a beautiful form and He caused it to be inhabited with mankind, animals, etc, and of course God Himself.

Isaiah 45:18:

¹⁸ For thus saith the LORD that created the heavens; God himself that formed the earth and made it; he hath established it, he created it not in vain, he formed it to be inhabited: I am the LORD; and there is none else.

When was the flood?

The flood happened about 1,650 years after Adam was created. The flood covered the whole earth and it affected the earth’s atmosphere, the land, etc, killing all not inside of Noah’s ark (except fish and other creatures who lived in water naturally, of course). Afterwards mankind, animals, etc, were not able to live such long lives as they had done before the flood. Refer to Genesis chapters 6-8.