alternatehistory.com

For the U.S., the longest, realistic hypothetical one I could think of was this:

Reagan/G.H.W. Bush (1981-1989)
G.H.W. Bush/Quayle (1989-1993)
Bill Clinton/Gore (1993-2001)
G.W. Bush/Cheney (2001-2009)
Obama/Hillary Clinton (2009-2017)
Hillary Clinton/Brian Schweitzer (2017-2025)
Chelsea Clinton/Tim Ryan (2025-2033)

52 years in total with Bushes and Clintons continuously either as President or as VP.
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