Sure. Fifteen minutes a day, every day, probably won’t hurt you. Hours can, especially if the work you’re doing is highly repetitive, the muscles are given very little physical rest with constant playing and high muscle tension during that time, the motion is stringhopping, you have a predisposition and/or existing injuries, etc. Lots of factors.
Keep in mind also that “practice” is a super vague term. The most important kind of “practice” we deal with around here is acquiring a new skill you don’t yet possess. Learning to do a hand or arm motion you can’t yet do. By contrast, what we’re not talking about most of the time is taking a technique you already have and learning new pieces with it. That’s real work, for sure. But it’s a very different activity. If you’re talking about learning a new motion, let’s be clear that this is what we are talking about.
In general, I get the best results from casual and somewhat haphazard approach. Not sitting down for very long periods of time, a few minutes here and there, with lots of down time in between. I make no effort to play every day, including when I am trying to learn a new motion or skill. Just because life intervenes. And very often, I come back later in the week and I can do something I couldn’t do before. There is a lot of research on the need for time breaks. The “learning”, if you like, happens during these breaks. I don’t think it really happens when you’re actually “practicing”.
In short, try not to worry about this. Only play when you’re enthusiastic, do so for short periods of time, try not to be highly repetitive if you can help it, put down the guitar frequently and walk away, and evaluate by feel, sound, mirrors, and video whether you are really acquiring the new motion you’re trying to learn or just repeating stock phrases a lot with no improvement. That to me is the most natural-feeling way to go about this.