Pole flips take around 3,000 yrs to complete as the dissolution of the iron core recoalesces and starts to spin again.
Solar winds says you get mutations because no magnetic belts to protect Earth during the flip.
Also, the geologic records sez the flips weren't that long at the beginning of Earth, but are that long as it matured, with the age and stability of the Sun, now proven as the transmitter of gravitic waves the pumps the spin of the core of Earth.