I seem to recall that the Arriva Trains North 150/2s still had their original seating and layouts (with new upholstery) in the early-mid 2000s, the orange vestibules were still present. I'm guessing that would have been about 16 or so years.
Of course heritage DMUs, refurbed in the 1970s retained their interiors through to withdrawal in the 1990s. I guess loco hauled mk1s may have stayed more or less (as built) internally for some years. Minus the odd moquette refresh...
So I guess 15-20 years wasn't uncommon in the immediate past.
Since privatisation though the never ending chopping and swapping of franchises and rebranding makes 5 years + a long time, especially on the ECML!