I would wager that you have to be one of the best, but not necessarily always the best to have a chance to be enshrined eventually. You look at a player like Cris Carter, who played in the same era as Jerry Rice and Tim Brown. Carter never once finished better than 7th in receiving yards. In 16 seasons, he led the league in receptions once, three times in TDs. The QB situation wasn't always in the best. Carter had Rich Gannon, who struggled prior to his Oakland years, Warren Moon in his late 30s gave one impressive season, and Brad Johnson, who was solid but not spectacular. That being said, Carter was consistently great, but I doubt that he was viewed as the unquestioned best during any stretch of his career. That being said, he was nominated almost immediately. I personally feel that consistently being in the Top 3-5 is normally enough to get in eventually. It doesn't hurt to have an individual award and/or a ring obviously.