I agree with all the comments thus far. Another consideration is keeping the classes and the instructor fresh. Knowing when to take some time off and refresh both mind and body is key. Avoiding extra load (i.e. subbing extra classes) and keeping variety for mental and physical variety is important. Burn out is a real and dangerous part of teaching and it needs to be addressed and recognized.
I’ve been a GF Director for many years and generally speaking many new GFIs teach 15 classes a week with ease. Over time, it seems many dial the class load back to 8 – 10 classes per week and supplement their income in other areas (i.e. training)
I myself teach 15+ classes per week, with no issue. The trick is variety, and to remember that the instructor is there to guide and teach, not to get his or her personal workout. I became far more successful when I spent more time giving my clients personal attention during a group class, focussing on their needs and technique, as well as keeping them interested and entertained. Do this rather than doing every movement and rep with them at the front of the class. Walk around while you cue, make eye-contact, smile! A great instructor can do all of these things while they lead a fun, safe and effective class!
There are so many variables to consider. Are they all the same type of class. If she teaches different class formats, I don’t see the dilemma. Even if she is teaching the same format, she is teaching. Proficient instructors now how to gauge themselves pretty well. If she is one that must workout the entire class, she is going to burn out.
Having been there and done that, it just is not good on the body.
I have never seen a standard, and I think it would be impossible to control because instructors can teach at multiple locations plus could even teach on their own in small groups outside.
I assume that this instructor is teaching all those classes at your facility and I am wondering why – from a business perspective – you would let this happen. Even if this instructor were to be the most popular one on the planet, you are exposing your business very much if this person all of a sudden could not or would no longer wish to teach for whatever reason.
Another thought: some instructors have a propensity to over-training bordering on a compulsion to exercising. If this is an employee, you may want to find a way to address it.