I think it depends on the person and their needs and their goals.
What’s their physical state?
What’s their state of mind?
What’s their schedule like?
How much time do they have to invest?
Minimum of twice a week is what I usually suggest, and then add more if you feel able to. Overall, I’d say between 2–4 a week is ideal.
Every class is different. Some classes are more technical and you can do many classes in a week. Some classes require more cardio and you may need to build up to doing more of those.
Talk to your teacher about your specific needs, and striking a balance between training enough and overtraining or under training.
Remember, it is about being the best well-rounded fighter. Well-rounded fighters need rest too!