Superheroes have a very hard time staying dead; the vast majority of Marvel and DC Comics characters who've died tend to return to the land of the living soon after, usually a year or so. The returns are motivated by fan outrage, long-term plans, and of course profit (it would do no good to have a popular character who drives sales such as Wolverine suddenly sail off into the hereafter). There are, however, a small number of superheroes who stay dead for a decade or longer - but one manages to "win" this contest by staying in the ground for over 40 years.
This epidemic of eternal life among superheroes and villains wasn't always the norm. The death of Jean Grey in the X-Men's Dark Phoenix Saga was always meant to be a permanent end for the character (editor Jim Shooter disagreed with the original ending - removing Jean's powers - and felt that her murder of billions of innocents warranted her death). But Jean came back to life a few years later, and so did many others. Among fans of the X-Men, it is easier to list the mutants who haven't died as opposed to those who have (and were subsequently brought back to life).
The landmark comic book event Crisis on Infinite Earths in 1985 had two notable deaths that were meant to stick. The original Supergirl was tragically killed by the Anti-Monitor in the 7th issue, and Barry Allen - the second Flash - died when his body disintegrated after running at a speed he had never before reached. Both of these deaths lasted for quite a long time - but ultimately Barry Allen would return in 2008 in Final Crisis and Supergirl would be revived in 2016 with DC Rebirth. With the Flash dead for 23 years and Supergirl dead for 30, it would seem like the two DC heroes take the crown for the longest time spend six feet under - but there's one hero that beats them both.
Bucky Barnes is officially the comic book hero with the longest time spent between death and rebirth. When Stan Lee reintroduced Captain America to comics in The Avengers #4 in 1964, he tragically killed Bucky off-panel in a World War II flashback moment (Lee famously hates sidekicks). Bucky remained dead until 2005 when Ed Brubaker penned Captain America: Winter Soldier, which revealed Bucky was alive and working as a brainwashed assassin. While in-universe, Barnes had never died, the decision was clearly never Stan Lee's plan. Thus, in the real world, Bucky remained dead for an astonishing 41 years.
Among comics fans in the 1990s, the axiom concerning this situation used to be "No one stays dead in comics except Uncle Ben, Bucky Barnes, and Jason Todd." Since then, all three characters have come back to life (but only temporarily in Uncle Ben's case). Bucky clearly holds the record among superheroes, but others - such as Marvel's Gwen Stacy and DC Comics' Thomas and Martha Wayne - remain truly dead...at least until an adventurous writer chooses to revive them.