The last time a single party held the presidency for more than 12 years was when FDR won 4 times. Prior to that it was civil war reconstruction.
Keeping the white house from 2020-2040 would be the longest single party streak without one president occupying the office more than 8 years. 3 consecutive Democrats would need to win. Ignoring the Democratic Republican steak at the beginning of the 19th century, because at one point everyone ran as a democratic Republican regardless.