This came from December, 2018 FOCUS magazine:

Question: How far back in time would we be able to go and still breath our planet's air?

Answer: Today oxygen makes up roughly 21 per cent of our air, but it was virtually non-existent in Earth's early atmosphere. Soon after photosynthesis started 2.4 billion years ago, oxygen levels crept up to 1 or 2 per cent - if you were to breath is air, you would die almost immediately. Between 850 and 600 million years ago (just before the Cambrian Period), oxygen concentrations increased steadily from 2 to almost 10 per cent: still not enough for humans to survive on. Fast forward to 400 million years ago (Silurian) and you could just about breath but might feel dizzy and confused at about 16 percent oxygen. Around 300 million years ago (Carboniferous) oxygen levels reached a human-friendly 19 per cent and have not dropped below since.