How Long Should I Train My Horse Daily?

the dial up or down on a horses fitness.
The high intensity workout can be cardio or strengthbased and occasionally include elements of both. It should be designed based on the horses performance goals. For example a dressage horse might do a demanding cavalletti workout to develop propulsive strength whereas a threeday eventer might do gallop intervals to increase aerobic capacity and stamina. A general rule is that the harder the workout is the shorter it is. In short if you plan to work your horses muscles hard in a strength effort you do not do that for hours. This weekly workout serves as a consolidated hard effort followed by ample recovery it is a structured dose of positive stress on the body without breaking it down.
In addition to a structured weekly workout a long but relatively loweffort session is an integral part of most conditioning plans. The purpose of this is to improve fatigue resistance and metabolic efficiency. A general guideline is for the duration to be 50 percent longer than the other weekly sessions. For example if your regular outings last 40 or 50 minutes this longer low aerobic session would be 60 to 100 minutes or more. In the case of arena horses this is an ideal time to go for a trail ride. When increasing length of training any given day do not simultaneously add intensity or hard work. 
In summary if we use a dressage horse for an example the weekly training volume might look like this during the Building Phase
Monday 40minute arena ride
Tuesday 30minute lunging and groundwork
Wednesday 15minute warmup 20minute cavalletti or hill workout 10minute cooldown
Thursday 45minute arena ride
Friday 45minute arena ride
Saturday 60 to 90minute trail hack
Sunday Off