I read something awhile ago that claimed the poles reversed about every 60,000 years. Patches of northern polarity appear more frequently in the Southern Hemisphere ( and vice versa) until the poles flip.
One of the original evidences, also for sea floor spreading, was magnetic mapping in the Atlantic during WWII for ASW purposes.
If your wandering around in the middle of nowhere and find magmatic outcrop with neat little holes cored in it, its from “paleomaggots”, guys that study/survey the magnetic record frozen in the rock at cooling.
If you were to look at the magnetic direction of the crust on the Atlantic surface, it would look like a parquet floor. The direction would just change...without a lot of space in between. That points to a sudden shift, geologically speaking. “sudden” is a relative term I guess.