Not an instructor (yet ;) but massage therapy is also a pretty physical job that involves a lot of energetic exchange like teaching. Like you mentioned, everyone is different. If you are inclined to ask the question, it may be that you are already doing too much or are headed towards it. Burnout is a real thing with legit stages. We went over it a lot in sports psychology, but wiki has a good summary of it, both the psychology listing of burn out as well as occupational burnout would be relevant to your situation.
Besides pain being an indicator of doing too much (that is when I've caught it FAR too late), my other main indicator is apathy/changed attitude. I know deep down that I love what I do, so if I start to dislike it or not be excited about it, I've learned that that is a sign I'm taking on too many clients/too many hours. The actual physical number depends on what's going on in my life at the time too. People who are under a lot of stress in multiple areas of their life experience burn out sooner, and are more prone to injury. Interestingly, the natural level of anxiety in a person also tends to be a predictor of the likelihood of injury (which, imo, at least partly explains why so many people get injured RIGHT before a competition).
I think it's also important to set boundaries BEFORE things start to get too crazy. Like set whatever is a realistic and easy goal for you and some you time, like "I will dance for myself one hour each week" (or whatever you choose). If your teaching load or general feeling of well being effects meeting that goal that should be a pleasurable release for you, it may be a sign that you are doing too much/there are some early signs of burnout happening that you might not be able to recognize. It can be really hard to recognize burnout when you're IN it. Part of why we went in depth about burnt out it for my degree is because it's often the job of the coach to recognize burnout cuz the athlete often can't recognize it. It goes back to the whole thing about a frog in boiling water just allowing itself to boil because the water changes so slowly it can't tell it's happening.
In others, I notice cynicism is a huge sign that they're working too much. And in sure when I got that apathetic/changed attitude feeling it came out as cynicism and I just couldn't be outside of myself enough to see it.
Dunno if that helps at all since its not about instructing! But yeah you can def tell when the instructor is just "over it" and trying to get through it cuz it's they're 7th class that day or whatever
