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?