At some point after the horse has developed good strong connective tissue and bone, the quality of the exercise is more important than the quantity. A young horse starting out need lots of long slow miles on a regular basis - preferably every other day. Working up to slowly adding more distance and more speed ( more trotting and less walking). This is a slow process and should not be rushed. The time required will depend on a lot of factors, the age of the horse, experience of the horse, conformation of the horse, etc.
Conduct a Standard Exercise Test
Put the horse at as close to a fixed rate as possible on the heart rate monitor, such as 140 beats per minute (BPM) for a fixed distance, say 5 miles and see how fast they do it. Record the time to complete the circuit at that set heart rate. This is always done on the same stretch of trail. As your horse gets into better condition, they will be able to complete the circuit faster, in a lesser amount of time, still at that same heart rate. Conversely, if they start to take longer to complete the circuit, then you are probably over training or have some other issue brewing, so more rest is indicated.
Adapted from a post by: