The shortest geomagnetic field excursion we know of was the Laschamp event, which happend from 42,200 to 41,500 years before present. All in all it took about 700 years: 250 years for the geomagnetic field to go for normal orientation to reverse orientation, then it remained reversed (at 25% strength) for about 440 years.
While the geomagnetic field was going from its original orientation to the reversed orientation it reached a minimum of 5% strength compared to the normal; and while at minimum strength it was chaotic, varying from place to place, so that it couldn't have been used for orientation.
This answer uses the popular term magnetic field strength for the quantity measured in teslas, which used to be called magnetic induction when I was in school and is nowadays called magnetic B-field strength (bleachhhh!).
A geomagnetic excursion is what geologists call a very short sharp geomagnetic reversal. In a full-scale typical geomagnetic reversal the field settles on the new orientation and stays that way about half a million years. In a short geomagnetic excursion the field reverses and then restores its original orientation is a very very much shorter time, measured in centuries or millennia. Short geomagnetic excursions occur more often than full scale reversals.
The Lascamp event is the quickest reversal ever measured in the geological record. Note that this quickest reversal covers the span of about 10 human generations for the field to switch from its original orientation to the opposite orientation, and then the reversed orientation endured for about 16 to 20 human generations.
In full-scale geomagnetic reversal it takes millennia (we think about 7000 years) for the field to switch orientation, and then it stays on the other orientation for hundreds of millennia. For comparison, the oldest written texts which we can read were written in Sumer about 5000 years ago, at the beginning of the 3rd millennium before the common era: a full scale geomagnetic field reversal takes longer than the entire history, from the deepest Antiquity to our present hectic days.
The Sumerian civilization was located in what the Greeks called Mesopotamia, the land between the rivers, and we now call southern and central Iraq.
But... During a reversal it is indeed possible for the field to fluctuate between the original and the opposed orientation. Some geologists even think that the field could switch orientation during a single human lifetime.
Wikipedia says that is some cases "the Earth's magnetic field is capable of shifting at a rate of up to 6 degrees per day". That would most definitely be noticeable by a human with a functional sense of magnetoception.
Detailed observation of the traces left by the full scale geomagnetic field reversal known as chron C5Cr (which happened about 17 million years ago) shows that during the switch of orientation the geomagnetic field goes through numerous excursions, switching orientation repeatedly over periods measured in years, before settling on the new orientation.
Note that during a reversal the strength of the magnetic field never goes to zero, it always remains at least 5% or 10% of normal; but instead of being nice and uniform all over the Earth, it becomes patchy, with multiple north and south magnetic poles.
Fun facts:
The first measurements of the strength of Earth's magnetic field about were made by Carl Friedrich Gauss about 190 years ago. Around the middle of the 19th century the field started getting weaker. It is currently at about 90% strength of what it was during Gauss's days. If it continues like this we will see the completion of a reversal in some 1500 years from now.
The northern magnetic pole, which for a long time was resident in Canada, has decided it wants to move to Russia and is currently racing across the Arctic Ocean at a speed of some 60 km (37 land miles, 32 sea miles) per year. (It is now located in the Russian part of the Arctic Ocean, or at least very closed to the boundary between the Canadian and the Russian parts).
