Spending countless hours playing a game may be a result of hardcore gaming, but is not a requirement. Simply playing a game without striving to improve or master the game is not hardcore gaming. That would be casual gaming.
After or near mastery of the game, the player can be called a hardcore gamer for the said game.
Mary: Why is Danny repeatedly playing the same racetrack over and over with different cars? There are many more tracks in that game!
Austin: Well, isn't obvious that he's trying to learn the track and find the car best suited for that track? You don't know much about hardcore gaming, do you Mary?
The people who do this are generally called "gamers" or "hardcore gamers", and are guaranteed to be virgins until the age of 42. They then defend themselves on game forums (not in real life because they have no one to talk to, hence the word, "loser") by claiming gaming is a sport and guys like Walshy are "cool".
Any one who has been outside in the last week hates hardcore gamers.
Normal Person Who Steps Outside Every Once in a While: Dude, get a fucking life.