I’ve just noticed that in the description of the Manual of Arms proficiency, it says that taking Manual of Arms and Riding once each allows the training of light cavalry (3 months) and Manual of Arms twice plus Riding once allows training of heavy cavalry 6 months).
So what is required for, and how long does it take to train medium cavalry?
If the cavalry are going to fight as Formed Mounted then Manual of Arms is required twice. If the cavalry are going to fight as Loose Mounted, then Manual of Arms is required just once.