In Calvin's Commentary on Genesis, he spends some time dealing with the question of how long Adam and Eve were in the garden of Eden together before they sinned. He writes:
The opinion has been pretty generally received, that they fell on the day they were created; and, therefore Augustine writes, that they stood only for six hours. (commentary on Genesis 3:6)
I haven't been able to confirm that Augustine actually wrote this. I have found mention of six hours in Dante's Paradiso (Canto 26), but so far nothing from Augustine.
Did Augustine say six hours, and if so, where?