I used to binge on gaming in my student years. It was glorious. Playing RPGs in particular could keep me in front of the screen for most of the day. Now I have a backlog of games.
What you or her friends think of as being 'really great places' might not be to her taste, just as her gaming isn't to yours.
15 hours a day is a lot, but if it's a rarity during the holiday and she's not missing work or college, there's really not a problem.
I agree with all of the above, I played video games for as many hours as I could get away with as a teenager, which on some days in the holidays would be all day! My parents moaned a bit at the time, but it didn't stop me getting good grades, then 2 degrees and a decent job. I still play games now, and still like the occasional all-day binge but I probably only get the chance a few times a year. I don't agree that spending all day doing one thing automatically equals addiction unless there are other factors involved.
15 hours is a hell of a lot in one day, but presumably there aren't that many days where she gets the opportunity to do this! She is holding down a job and going to college. Is she playing the games online against friends?
OP, if you want to take it up with your DD, first make sure you are clear in your head exactly why you are not happy. Is it because it's something sedentary, because she's having a lie-in until late morning, because it seems antisocial, because 15 hours doing one thing seems too much (though would you still feel the same if it was a really long day out?) Or do you just not like video games? There really are worse hobbies, and gaming is brilliant downtime from work etc because it is so absorbing, your mind has to concentrate on what you are doing instead of stressing about life.