This depends on many factors and, above all, the horse's metabolism. It can take anywhere from 3 days to about a month to clearly see the effects. However, there are always other factors, such as changes in weather, coat change, grass composition, etc., that can affect the horse's metabolism. Nervous horses usually become calmer, while more "phlegmatic" horses become more alert and energetic. So there is usually a positive effect towards the “inner center” in both weight and temperament.