It's a combination of devotion to the sport (watching mass amounts of fights, learning the history, closely following current happenings) and appreciation of all facets of the sport (don't think as many fights are boring). That's my opinion. I don't think there's a time qualification, though you're naturally going to need some time to acclimate yourself with the requisite knowledge, but I think 1-2 years is definitely enough time to transfer yourself into a hardcore fan. I feel I did that back in 2005-2007. I feel like I went from "TUF noob" to looking up Shooto/SuperBrawl/KOTC fights on the internet.
My interest in the sport has waned a bit, but I still consider myself a hardcore fan because my understanding of the sport and my opinions/perceptions are formed on a more advanced level.
Different people are likely to have different definitions, but if you're following MMA outside of the UFC and Bellator and you're making an effort to soak up as much knowledge as you can, I think you can call yourself a hardcore fan.