Small scratches normally heal very quickly, within 3 to 7 days, if the skin is kept clean and dry. This is because the skin is constantly producing new cells and if it is kept clean, the new cells are able to cover the wound quickly. On the other hand, if the skin is infected, it takes longer to heal. In most cases, horse scratches will heal in up to two weeks but if the wounds are deep or infected it can take much longer. In this case it's important to clean and treat the affected skin with an antibiotic cream every two to three days, until the infection clears completely.
It's also a good idea to keep your horse indoors, in a dry and clean place, until he is healed, especially during hot, rainy weather. This will prevent further aggravation of the wounds.