I think you have to differentiate between legends and cult heroes.
Big Dave is a cult hero because of the difference he made to the team, and scoring the opening goal against Palace.
Chris Brunt has to be a legend because of length of service, sheer number of games, and the number of assists he's put in. He dictated our attacking play for several years, no matter which position he was in. Helped to reestablish us in the top division.
Similarly, I'd call Sneekes a cult hero, but Bob Taylor a legend. It's not just length of service either; for me, Zoltan Gera is a cult hero, Kevin Phillips a legend.
Remember, Laurie Cunningham was only with us two years, and he's revered.