St. Augustine explains an acceptable interpretation in Book 6 of his “Literal Meaning of Genesis.”
He explains that the six days represent not literal days, but a scheme or plan of creation. The actual creation during those “days” was instantaneous and of things in potency and causation, but not necessarily their final visible form which would be shaped later over time. For example, he places the actual formation of man’s body after the seventh day (which explains why there is two creation accounts of man in Genesis):
There can be no doubt, then, that the work whereby man was formed from the slime of the earth and a wife fashioned for him from his side belongs not to that creation by which all thing were made together, after completing which, God rested, but to that work of God which takes place with the unfolding of the ages as He works even now.
This interpretation works well with concepts like an old universe, the big bang, and evolution–ie God created all things at once in potency (the big bang) and then formed them over time (old universe, evolution). St. Augustine compares this formation of things to how mountains and rivers are shaped over time.
Even with man, Genesis doesn’t say how long it took God to form man from the slime of the earth after the seventh day–but His rational soul was immediately infused when God breathed into the body, as it is immediately infused in each of us.