(1) St. Augustine did not speculate that God could have created Adam "over a long period of time." I could be wrong - St. Augustine wrote about forty volumes of material and I haven't read it all, but I'm not buying this claim without specific citation.
(2) There is no such thing as a strictly scientific question, and the Church has indeed taught the faithful definitively on monogenism.
There are scientists who support polygenism and it is indeed a scientific question.
William Greenough Thayer Shedd's Dogmatic Theology, Vol. 1: "Respecting the length of the six creative days, speaking generally, for there was some difference of views, the patristic and mediaeval exegesis makes them to be long periods, not days of twenty-four hours. The latter interpretation has prevailed only in the modern church. Augustine, teaches (De Genesi ad literam, IV.xxvii.) that the length of the six days is not determined by the length of our week-days."
More recently Oliver Barclay, wrote in a conclusion to a published creation/evolution debate that:
"...many on both sides of the discussion will agree that the most natural reading of Genesis 1 is in terms of creation in six 24-hour periods. That, after all, is how it has normally been understood in the history of the church until quite recently. There are exceptions, like Augustine who thought it referred to a long process, and he had considerable influence, but at least since the seventeenth century most people have understood it in terms of six periods of 24 hours, until modern geology got going in the early nineteenth century (before Darwin)."
Bernard Ramm writes:
"...The point Augustine actually makes is that the creation days are so great, so majestic, so profound that we cannot consider them as mere sun-divided days but as God-divided days. They are creative days, not solar days, and so he calls them natures, growths, dies ineffabiles."
Here's the three sources:
William G.T. Shedd, Dogmatic Theology, Vol. 1. (Edinburgh: T.& T. Clark, 1889), 475-476.
O.R. Barclay, “Summary and Conclusion,” Derek Burke, ed. Creation and Evolution. Where Christians Disagree, 1985. (Leicester: IVP 1986), 269-270.
Bernard Ramm, The Christian View of Science and Scripture. (Grand Rapids: Eerdmans, 1954), 147.
You're right about the Church rejecting the scientific hypothesis of polygenism. The Church definitively teaches monogenism: that the whole human race belongs, physically, to one family.