Longest time between a superhero's death and revival

"Death" in comics doesn't have the permanence it does in real life. The more popular a character is, the harder it is to kill them and leave them dead. Of the many, many superheroes to have fallen, only to return later, Marvel's Bucky Barnes holds the record with 37 real-time years between his death in Avengers vol. 1 #56 (1968) and his return in Captain America vol. 5 #1 (2005).

Bucky was considered dead until his entire history was retconned by Ed Brubaker in 2005 to reintroduce him as Winter Soldier.