In the calvin cycle, glyceraldehyde-3-phosphate (G3P) is first and foremost responsible for making glucose. To make one G3P molecule, you need three turns of the calvin cycle. In the GP3 molecule, you hade 3 fixed carbon atoms. So to make a six-carbon glucose molecule, you need two GP3 molecules. Therefore it takes 6 turns of the calvin cycle (photosynthesis) to make a glucose molecule.