12-15 hours as a one-off bender (like a weekend deal), you're probably OK as an occasional deal.
As a daily deal, that's not healthy... even for professional Twitch streamers. Even if you run traditional business work schedules (like a 15 minute break every 2 hours & an hour lunch at 4 hours), it gets a bit exhausting after 8 hours (& I used to work 4 10-hour shifts at one job... which puts a cramp on your social life during work days, even with somewhat reasonable 30-minute commute).
I would recommend breaking up your gaming activity with some a bit more physical activity, unless you're playing a highly physically active game (like DDR, Just Dance, Wii Fit, ect.), just to mitigate the potential health risks incurred by prolonged sitting. Even something as simple as just taking a moderate stroll through a park or even a mall for like 15 minutes every 4 hours (at a bare minimum, every 2 hours is recommended) would be beneficial.
The only time doing 12-15 hours per day would be acceptable is if you're training for a gaming marathon (like trying for a world record OR even attempt an ironman run... where most gamers who prepare for such attempts tend to struggle after 72 hours, even with scheduled breaks within the run).
Moderation & a good person / social / work balance is key for a healthy life.