I remember seeing a doco on this ( and how they were working out the timings ) a few years back
There is a large crack in the mid atlantic that is increasing in size and has been for a long time. By studying the polarity of the volcanic residue that came up, as you move away from thre crack, you can see what has happened.
https://www.geolsoc.org.uk/Plate-Tec...Atlantic-Ridge
No idea how quick the flip is tho.
Andrew