IndigoChild5559
Loving God and my neighbor as myself.
I'm going to answer only for myself, as Jewish opinions on this vary so greatly. Although I'm observant (shabbat, kosher, holy days, etc.) my theology is extremely liberal. The discoveries of historians, archaeologists, and literary analysts weigh very heavily on my views. I find it meaningful to turn the Torah over and see it from many different angles, secular scholarship being one of them. Before I directly address your question, we need to review some necessary background.@Sargonski
The Tanakh holds the Sabbath observance central to Jewish monotheistic identity? From quite early times?
The Tanakh can roughly be divided into two different ages. There are bronze age stories, and iron age stories.
With regards to the people and events in the bronze age stories, we do not find any corroborating evidence from outside sources. There are no ruins of Sodom and Gomorrah. There is no indication that Canaanite cities were conquered by Israelites. There is no Egyptian record of a Jewish exodus. That doesn't mean these stories didn't happen. But it would suggest that whatever actually happened got edited and embellished over time as the stories were orally passed down, and finally put into writing by iron age authors and editors.
The iron age stories, on the other hand, do have corroboration both from archaeology and from outside texts of that era. We know for example that Hezekiah was a real historical person. We know that the Northern Kingdom of Israel and the Southern Kingdom of Judah both existed. We know that the Jews went into captivity during the Babylonian empire. You get the idea.
So with that foundation, let's return to your question about when the Torah came into existence and when did keeping the Shabbat become the norm for Jews.
Jewish Shabbat keeping IS referred to in sources outside the Bible. The oldest so far is the Elephantine Papyri from the 5th century BCE.
I accept what textual analysts say about the Torah having many authors, whose works were later spliced together to form the Torah as we have it now. There are general themes that are so entrenched that they likely predate any text at all, the seven day week being one likely candidate. When the original texts were written is unknown, but the splicing seems to have happened over the course of several centuries. Most scholars say this happened during the Babylonian captivity, although there are some who believe the process started as early as reign of King Josiah of Judah in the 7th century BCE. The process was finished sometime during the Persian period.
However, having the texts and observing the laws are two very different things. Even the Biblical stories are replete with incidents of disobedience, violence, lack of compassion for the poor, and rampant idolatry. I realize that the absence of evidence is not evidence of absence. Nevertheless I find it notable that the first archaeological evidence that the Torah laws were well known and widely observed didn't happen until the Maccabean era.
So, although we can't know when Jews first began keeping the Shabbat, we know that it goes back at least as far as the 5th century BCE, and it was certainly practiced on a wide scale by the Maccabean era.