I'm not sure how much is "often" or "heavy" and how much is "occasionally" or "moderate." My SO plays video games. Sometimes it's hours and hours of Gears of War or something like that with his friends. Sometimes after a hard day, he'll plug in his headphones and sit on his new desktop (that he built...for gaming...) and just "fuck shit up" for a couple hours. Some days I don't even know if he games at all. Some days he just watched stupid youtube videos.
The truth is, I think there are more productive things to do than play video games, but it's also not mine to decide. He usually spends ample time with me. He keeps up in his courses. He works a full time job over the summer. If I feel like I'm not getting enough time with him because he's gaming a lot, I'll make the time with him. I can't play for shit most games he likes, but I'll lay in his lap and watch him play. This upcoming school year, I've got a new apartment so I'll be able to make him and his friends food while they game over at my place. It isn't what I'm personally into, but it's just another hobby or interest. It's a part of who he is, and if he likes it, and if it isn't overtaking his life, I'll support him, just as he supports me with my "silly" hobbies.
Sometimes it annoys me when he has been choosing to spend any free time gaming and I haven't seen him much in a while. Sometimes when he's had a shit day and so have I, he'll go to games and I'll want to go to him but can't. Then, yes, it bothers me a bit- but it's not something IMO to argue about. When he pauses his games to take a break for food or the bathroom I'll just tell him I need some time and he'll take a few minutes to dance with me or talk with me.
12 hours is excessive- not because he's video gaming, but because it's just excessive. Nobody should do anything for that long unless it's a rare thing- like once every few months having a weekend devoted to gaming. If it's verging on the edge of addiction- that's a problem. If gaming takes precedence over work, that's a problem. If gaming WANTS come before your NEEDS, that's also a problem. But the problem might be you if you're pissed because it's *gaming.* It's not your job to monitor your SO's activities and hobbies unless it is something actually harmful to them or to you.