Another person answers fairly accurately. The answer is around 500 million years. (Adapted humans can survive on a bit less than 10% oxygen, though, so could go back further.)
There was some stupid book I was reading where a team went back a billion years into earth’s past with no protective gear. The O2 percentage would’ve been around 2.5 percent then at sea level, meaning death within 5 minutes or so.
I did not keep reading that book.