I use it at least once a week, every time the season changes. It boosts my old mare, it's a really good thing for her intestinal transit (she has often colics). And for my younger one, I give it usually as a reward after an intense effort. In winter, I give it once every 2 weeks :)
The mash helps to "help" your horse's digestion. Its boost effect and its refresh action is useful whenever from the moment the horse isn't healthy, or stressed, exhausted... It stimulates the appetite and it's particularly good for old or too thin horses. It's recommended for horses who're going to change their food, and horses who're often suffering colics.
Be careful for horses who're already too fat who don't work, it can cause foundering. Some people also give it before a competition. That's a good thing but be careful to don't use it too much!