Hugh Binning, Works, p. 91:
But why, then, did he take six days for his work? Might he not with one word of his power have commanded this world to issue out of his omnipotent virtue thus perfect as it is? What needed all this compass? Why took he six days, who in a moment could have done it all with as much facility? Indeed, herein the Lord would have us to adore his wisdom as well as his power. He proceeds from more imperfect things to more perfect, – from a confused chaos to a beautiful world, – from motion to rest, – to teach man to walk through this wilderness and valley of tears, this shapeless world, into a more beautiful habitation; through the tossings of time, into an eternal Sabbath of rest, whither their works shall follow them, and they shall rest from their labours.
Source: http://www.puritanboard.com/showthread.php/88956-Why-six-days-for-His-work, Comment #1