I'd say 15 is a long short these days, but I could be wrong.
The story being told, the quality of production and how captivating the acting would be few factors that come to mind. If I click away to something else online, then it *might* be too long. But if I'm in a festival setting (trapped!) I'd hope it hits a lot of the marks. I want to feel taken care of.
The tidier it is, the better the chances it won't have people checking text messages or the time.
Pack a punch. Make it count. Keep asking yourself how to tell the story with less.
2 cents (plus interest).