This is a tricky question to answer because it depends on a number of different factors, for example the time of year and the climate you live in,  the type of horse you have, whether the horse is being stabled or living out at grass  and how much work the horse is doing. So to give some examples. A shaggy coated pony living out all year round in the United Kingdom (where I am from) will need his coat left on as protection against the weather in the winter and will shed his coat ready for the warm summer weather (if it ever comes!) .  A competition horse which is exercised hard every day and living in all year round may have a full clip and might have to be clipped several times during our winter otherwise they get very hot and sweaty if the coat is allowed to grow long. Then he will be rugged up to keep him warm when not being ridden. Those are the two extremes but there are variations in the amount of hair which can be clipped off to keep the horse or pony comfortable on a particular work programme and management system. Hope that helps and good luck with your dream of becoming a vet.