I don't think being a hardcore gamer has anything to do with the amount of time you put into gaming. You can be a hardcore gamer and still have a full-time job and social life.
Whether you are a hardcore gamer or not depends on the types of games you play, at least where I come from. Are you into 2D Fighters/Technical 3D Fighters, Mario Bros.-style 2D Platformers, RPGs (Tactical, Japanese or Western), or Shmups? Then you are probably a hardcore gamer.
A casual gamer is one who sticks to the easy-to-understand, don't-spend-time-on-it type of games. First-Person Shooters, Mario 64-style 3D Platformers, Mobile Puzzle Games, and any Wii game that thrives based on how much waggle is involved: those are the casual games.
A hardcore gamer can play casual games and still be hardcore. A casual gamer can not play hardcore games and still be casual.
Of course, if a gamer puts too much time into a 'in-between' game, then they'd be hardcore. If you just play an FPS or Fighter online for the fun of it, your a casual gamer. If you play those same games with a close watch on your Win/Lose ratio and consider making a brand new account if you have a bad day, you'd be hardcore. Play "I Love Rock & Roll" on Easy? I'd question whether you've even seen a video game outside of your iPhone. Play "Through The Fire And Flames" on Expert? I'd question whether you've every seen the outside world.
So, maybe it's just a mental thing.
Anyway, I consider myself hardcore, with a casual attitude. I don't care (too much) about Win/Lose ratios or getting 100% achievements/trophies, but I still play shmups, Tactical RPGs, 2D Fighters and other genres that Father Time has left in the dust. I also take gaming very seriously, wanting it to be as aesthetically pleasing and immersible as possible.
C'mon, I come to this site, don't I? I'd say that any gamer who actually prints covers out for their games on quality print paper and goes out of their way to get cases for games so that their games can look really great on their shelves has far crossed over the line into hardcore country. Most gamers I know just keep their CD-based games in CD Folders and their cartridge-based games in boxes. Probably in the attic/basement.