Do we have any information on how common it was for someone to believe religion as purely fiction in ancient times? Did humans just at some point start to doubt the veracity of religious texts or were there always people thinking "nah, this is just metaphors"?
This depends what you mean. Socrates was executed for being an atheist; literally not believing in the gods of the state. But he did believe there was some divine entity he called a daimon that warned him not to do things. Plato has him go through all this in the Apology. Socrates' argument there is while he doesn't believe in these gods, he does believe in something.
Euhemerus didn't believe in the myths about the gods. He argued that Zeus was really a king of Crete (if I remember rightly), and over time the myth of godhood formed around him. But that's not an explicit rejection of a divine being; just the myths associated with Greek religion.
Epicurus was probably the closest to what we call an atheist today. He thought humans were entirely matter, i.e. there's no divine spark in us. There's no afterlife. He had an atomic theory of the universe, in which atoms fall through space and by coming in contact with each other create all the things in the physical world.
He argued, and I love this argument, that the mind must be material, because wine doesn't just effect the operation of the body, but also of the mind. A material thing should only interact with another material thing (this is from Plato) and so the mind must be material.
But he still said gods existed; they just don't give a fuck about us or our lives.
Later on, you find Neoplatonists, who develop an idea of a single, unchanging, unmoving One, from which all existence originates. I don't know what you do with a single, unchanging, unmoving entity as far as religion.
Weirdly enough, these guys were studying and corresponding with early Christian scholars, which might explain some of the weird stuff that happened around the doctrine of the Trinity. It seems early Christians were trying to fit the Gospels into that Neoplatonic mode of thinking. It's well-known that when Erasmus produced his edition of the New Testament, he didn't include a reference to the Trinity, because no text to support that existed. That doctrine is a product of the early Christian scholars, who were studying and working with those Neoplatonists. (When the Pope complained about the exclusion of the Trinity from Erasmus' edition, and he replied that no text supported it, so goes the story, the Pope forged one, and Erasmus put it in his next edition.)
Christopher Hitchens curated and published a collection of what he considered Atheist writing from the time of Lucretius (the major source for Epicurus) to Dawkins. The Portable Atheist.
If we take that as a survey of atheist thought, we get Lucretius, and through him Epicurus, so 3rd and 1st C BCE. Then Omar Khayyam, 12th C CE. Then Hobbes, 17th C CE, and then a whole string of other thinkers from there, Spinoza, Einstein, Shelly, Mill, Twain, Lovecraft, Mencken, Sagan.... it's a long list.
There's a long gap between Lucretus and Omar Khayyam, and then another long gap until Hobbes, and then you start getting more and more outspoken "atheists" - at least as Hitchens judged them.
I don't know which of these thinkers and authors I've discussed you consider "atheist," so I can't give you a definitive answer. But I believe you can see a development of atheist thought and the time spans involved. I hope that helps in some way.
There were, in fact, atheists and agnostics in the ancient world, but not everyone who was called an "atheist" in antiquity would necessarily be considered an "atheist" today.
The modern English word atheist is derived from the Ancient Greek word ἄθεος (átheos), which is formed from the noun θεός (theós), meaning “deity,” plus the prefix ἀ- (a-), meaning "without." The ancient Greeks and Romans, however, routinely used this term as an insult and snarl word for basically anyone who went against any kind of traditional religious ideas. Many, if not most, of the people to whom the word was applied in antiquity did actually believe in the existence of deities of some kind.
Take, for instance, the early Hellenistic Greek philosopher Epikouros of Samos (lived 341 – 270 BCE), who taught that deities exist, but they have no involvement in human affairs whatsoever. Instead, he held that deities are ideally perfect beings who live separate from humanity in a state of perpetual ἀταραξία (ataraxía) or "untroubledness." Ancient Greeks and Romans from Epikouros's lifetime onward regarded him and his followers as archetypal ἄθεοι, even though they explicitly and emphatically affirmed the existence of deities. Pagan Greeks and Romans even accused early Christians of being ἄθεοι because they refused to worship the traditional deities of Greek and Roman religions.
That being said, there were some people in the ancient world whom most twenty-first-century people would consider atheists or agnostics. Such people were rare—but they are attested in the historical record. Probably the clearest and most famous example is the Greek Sophist Protagoras of Abdera (lived c. 490 – c. 420 BCE), who wrote a treatise titled On the Deities. This treatise has not survived, but the opening line has thankfully been preserved through quotation by the Greek biographer Diogenes Laërtios (fl. c. third century CE) in his Lives and Opinions of Eminent Philosophers 9.8.51. It reads, in Ancient Greek:
“περὶ μὲν θεῶν οὐκ ἔχω εἰδέναι, οὔθ’ ὡς εἰσὶν οὔθ’ ὡς οὐκ εἰσὶν οὔθ’ ὁποῖοί τινες ἰδέαν· πολλὰ γὰρ τὰ κωλύοντά με εἰδέναι, ἥ τε ἀδηλότης καὶ βραχὺς ὢν ὁ βίος τοῦ ἀνθρώπου.”
This means, in my own translation:
“Concerning deities, I cannot know whether they exist or not, nor can I know of what sort they may be; for many things prevent me from knowing, namely the obscurity of the subject and the brief life of a human being.”
This quote qualifies as a statement of agnosticism by any reasonable definition. Indeed, it is practically a dictionary definition of agnosticism. Tim Whitmarsh, the A. G. Leventis Professor of Greek Culture at the University of Cambridge, in his book Battling the Gods: Atheism in the Ancient World, published by Penguin Random House in 2015, goes even further and argues on pages 88–91 that this quote is a statement not merely of agnosticism, but agnostic atheism, since Protagoras is famous for his statement “πάντων χρημάτων μέτρον ἐστὶν ἄνθρωπος” (“The human being is the measure of all things”), which implies that anything it is not possible for a human being to know about cannot be said to exist at all.
In a slightly different vein, the Greek Sophist Prodikos of Keos (lived c. 465 – c. 395 BCE), who was a younger contemporary of Protagoras, argues that the deities are really just personifications of natural phenomena invented by humans. He expressed this view in his fragment D-K B5, which has been preserved through quotation by the much later Pyrrhonic Skeptic Sextos Empeirikos (fl. c. late second century CE) in his Against the Logicians 9.18. The fragment reads:
“ἥλιον. . . καὶ σελήνην καὶ ποταμοὺς καὶ κρήνας καὶ καθόλου πάντα τὰ ὠφελοῦντα τὸν βίον ἡμῶν οἱ παλαιοὶ θεοὺς ἐνόμισαν διὰ τὴν ἀπ’ αὐτῶν ὠφέλειαν, καθάπερ Αἰγύπτιοι τὸν Νεῖλον καὶ διὰ τοῦτο τὸν μὲν ἄρτον Δήμητραν νομισθῆναι, τὸν δὲ οἶνον Διόνυσον, τὸ δὲ ὕδωρ Ποσειδῶνα, τὸ δὲ πῦρ Ἥφαιστον καὶ ἤδη τῶν εὐχρηστούντων ἕκαστον.”
This means, in my own translation:
“Ancient people regarded the sun and moon and rivers and fountains and, in general, all the things that benefit our life as deities on account of their benefit, just like the Egyptians regard the Nile; and, on account of this thing, they regarded bread as Demeter and wine as Dionysos and water as Poseidon and fire as Hephaistos and, in this way, each of the things that benefit us.”
These are just a couple of the clearest examples of ancient thinkers who might fit the modern definition of agnostics or atheists. For more information, I would highly recommend reading Whitmarsh's book, which is superbly well written and accessible and does a fantastic job of marshaling evidence for irreligiosity and religious skepticism in the ancient world, although I do have some criticisms of it. In particular, I think Whitmarsh tends to use the term "atheism" too broadly to refer to basically any kind of irreligiosity or skepticism toward any kind of religious ideas and/or practices and I think he calls a lot of ancient figures "atheists" who probably wouldn't fit most people today's definition of the word.
I also think he greatly overestimates the extent to which atheism and religious skepticism pervaded ancient Greek and Roman societies in general. At one point (on page 230), he even leads his readers to the impression that the Roman Empire in later antiquity just before the rise of Christianity was within reach of possibly becoming a fully secular society in the modern sense. I think that this was nowhere even close to the case.
In reality, all the evidence overwhelmingly indicates that Greek and Roman societies, even in late antiquity, were deeply infused with religiosity of various kinds and the vast majority of people in those societies wholeheartedly believed that deities are real supernatural beings with distinct personalities. Those like Protagoras and Prodikos who doubted the literal existence of the deities were only a small minority.
Others have mentioned the western world, I'll try to cover India.
Indian philosophy can be divided into two- Astika and Nastika. Nastika were philosophers who disagreed with the teachings of Vedas. Carvakas were one of the most important atheist schools among the Nastikas. There are several lengthy attempts to refute the philosophy in Buddhist and Astika texts. The Carvakas believed in Hedonism and denied the existence of soul or God. They only believed in direct perception and rejected metaphysical knowledge.
Buddha believed that a creator God was irrelevant in the ultimate goal of attaining Moksha and Nirvana. Although it could be something closer of agnosticism, it can't really be called that strictly.
Jainism rejects the existence of a God completely. Though, Karma and reincarnation are both an integral part of Jain philosophy.
Check out this previous thread. In particular the comment by /u/NumisAl