Geophysicists measured a magnetic reversal happening 16.7 million years ago through centuries of lava flows. The field doesnt flip suddenly (like every 22 years on the Sun), but weakens and flickers over several centuries before reorganizing in the opposite direction. A magnetic compass would not be useful for navigation during this period.

Computer simulations of the geomagnetic dynamo also sees flickering, both with and without a reversal. Its possible we are merely in a minor flicker now or in the early stages of a full reversal. The intensity of the Earths field is down by 8% since first measurements about 200 years ago. There are claims of a stronger field millennia ago measured in pottery made from high iron clays. They would magnetize slightly after cooling off.

https://academic.oup.com/gji/article/186/2/580/587671