But then why 6 * (some period of time). simply some period of time would suffice.
In Qur'an, it is said that heavens are formed in two stages(2 spans of time) and the Earth is created in two stages(2 spans of time) and also, living organisms are created in tow stages(2 spans of time). 2+2+2 = 6 stages.
Also, it is said in Qur'an that the raw material of heavens was something like smoke and heavens are created from this smoke.In addition, the number of them is seven and they was singular as one heaven, then became seven. As well, all planets and stars that we see are in the first heaven. Two stages of heavens formation maybe when they were smoke and when they became condensed.
Also,two stages of the Earth creation maybe when it was very hot and molten and when the Earth’s crust became cool to become habitable.
As well, two stages of the creation of the living organisms maybe when there were just prokaryotes in the Earth and when the eukaryotes were created.
41:9-12 " «9»Say: What! do you indeed disbelieve in Him Who
created the earth in two periods, and do you set up equals with Him? That is the Lord of the Worlds.«10» And He made in it mountains above its surface, and He blessed therein and ordained therein its provisions, in four periods(seasons) alike for the seekers.«11» And He applied His design to the
heaven,
which was [as] smoke; He said
* to it and to the earth, “Come [into being], willingly or unwillingly!” both responded, “We do come in obedience.” «12» So
He completed them as seven heavens in two periods, and He assigned to each heaven its command. And We adorned the lower heaven with stars, and (provided it) with guard(not to collapse). Such is the Decree of (Him) the Exalted in Might, Full of Knowledge.
* This conversation is not like our conversation. It illustrates the power of God.
Remain two periods :
32: 4 " Allah is Who has created the heavens and the earth,
and all between them, in six periods ".
This verse states that the earth and the heavens and all between them are created in six periods. Previous verses state that God created the heavens in two periods and the earth in two periods. Consequently, all between them( living organisms ) are created in two periods. Also this verse can indicate that there are living organisms in other planets besides the earth.