In the first chapter of Genesis, it gives us a description of God creating the heavens and earth and it says that on the seventh day he had finished his word and had rested (Genesis 2:2-3), but did any Early Church Father believe that the Creation account in Genesis was allegorical and not literal?
According to this article:
Clement of Alexandria states that "That, then, we may be taught that the world was originated and not suppose that God made it in time, prophecy adds: 'This is the book of the generation, also of the things in them, when they were created in the day that God made heaven and earth' [Gen. 2:4]. For the expression 'when they were created' intimates an indefinite and dateless production" (Miscellanies 6:16 [A.D. 208]). While this might seem like a claim for long ages, other sources indicate that Clement held to instantaneous Creation such that "indefinite and dateless" properly refers to an immeasurably short interval.
Origen, however, is well known, even in Creationist circles, as denying that Creation took six days. "I do not suppose that anyone doubts that these things figuratively indicate certain mysteries, the history having taken place in appearance and not literally" (The Fundamental Doctrines 4:1:16 [A.D. 225]).
A claim is made for Cyprian in the mentioned article, but the quote is taken out of context. Further reading would be required to determine if he is speaking of Creation taking six thousand years, or if he merely holds the view of many others that judgment will occur six or seven thousand years after creation.
Augustine stated that "at least we know that [the Genesis creation day] is different from the ordinary day with which we are familiar" (Genesi Ad Litteram). However, he also believed in instantaneous Creation. This article is worth a read if you desire a more in-depth exploration of Augustine's beliefs.
This article from Answers in Genesis, which is of course strongly biased toward a historical reading of Genesis, affirms that Clement, Origen and Augustine all believed in an allegorical reading, so we can have high confidence that such assessment is correct. Still, it's worth noting that these are exceptions; the majority of the church throughout history accepted Genesis 1-11 as plain history. Certainly it is history steeped in additional allegorical meaning, but Scripture reaffirms time and again that God uses real events as symbols.