If all I ever knew were common composers of average skill and then I found a work like Bach's Mass in B minor, then the natural conclusion would be "This must have taken a lifetime to write!" I would judge it this because I see things through the eyes of common men - being I never knew of such a thing as a musical genius of the likes of Bach. Did Bach really spend a lifetime writing that Mass? No, but according to my good logic I would think it took the composer a lifetime to write something of such fine craftsmanship.
So if God is the Master Composer of all things (far greater than Bach) then wouldn't He be able to work in such a way as to make something appear to take much longer than what it actually did? Surely an Almighty Being could bend, stretch, and compress time and processes as He so desired. If 99.99999999% of all men cannot even compose as well as Bach, then isn't it beyond arrogance to think you really know the details of how the world was created?