Well, if you read about how the Earth's magnetic field is formed, you'll learn that the core field of the planet is actually quite weak and delicate, and that the swirling currents of the mantel and inner core are what amplifies it. Based on this, if the core source of the field were to reverse polarity, like say, by flipping around, it would take a long time for the field to reduce to near nothing then build back up again. A field of that size would resist a change in polarity for thousands of years until it finally realigned with the source.