Colin Russell, played by Michael Cashman, reappeared in Eastenders last year for the first time since 1989, so that's 27 years.On EastEnders, I think current villain Willmott-Brown might hold the record for that show, with 25 years between appearances.
Apart from the recast, didn’t they make his birth date 13 or 14 years earlier than previously stated?The longest between appearances in Emmerdale I can think of is Terence Turner, absent from 1985 - 2006, 21 years.
With Kenneth Cope, it was a 42-year gap. With Philip Lowrie, it was 43 years.Dennis Tanner probs, or Jed Stone, Minnie Caldwell's lodger from the 60s who showed up decades later as a homeless.

Terence’s date of birth was changed from the mid-1960s to the early 1950s.The recast Terence Turner in Emmerdale in 2006 was significantly older than he was in 1985.