Horses in their natural state are herd animals and are constantly on the move with the changing terrain and seasons, and this constitutes natural growth and wear of their feet (hooves).
In this man made world of ours, these animals are led, ridden and driven over terrain and areas that they would not naturally travel.
Their breeding, feeding, yarding and stabling has over time, weakened their normally strong feet to an extent, that over the last 300 or so years, with mans intervention from their "free" status, we have to shoe the modern horse to suit its particular work-sport capabilities to protect and support their feet.
Most horses being ridden or worked are shod to protect their feet.
Many animals that are not worked excessively, that are ridden on sand or dirt, or have sound and strong feet, can be left barefooted if trimmed correctly.
Providing a good depth of wall (rim) is retained with a rounded edge and no wing, the horse will remain comfortable.
There is no set time for reshoeing or trimming, as the condition, growth, and even the shape of the feet can determine this.
As most shod horses grow approximately 1 centimetre of hoof in 4 weeks, the need for reshoeing is at about 5 to 6 weeks intervals, some even sooner.
Unshod horses and ponies will "grow to the rate of wear", e.g., more growth in warmer, harder terrain, and less growth in colder, soft or sandy conditions.
The breed of the horse, as well as the feed, such as hard feed and grain, type of hay, lucerne, paddock grasses, the health of the animal, can often determine whether it needs to have shoes or not.
Most importantly, any trimming or shoeing should always be done by a qualified and competent tradesperson