Read this article, it is very informative http://www.aaep.org/info/horse-health?publication=2313
I like to mash when there are any risks of colic or dehydration, like traveling, competing, and when there are dramatic changes in the weather. But you have to be careful not to mash too often. It causes mineral imbalances that can actually cause metabolic conditions.
mine used to get it once a week: 1/2 red scoop for a pony, 1 red scoop for a horse (equal to 4 and 8 cups, respectively) of bran, and 1/2 cup of sweet feed for a pony or 1-4 cups of sweet feed for a horse depending on work level and normal grain intake. Spoon of salt too.
We give bran mash to horses who need sometimes. We just put bran in a bucket and soak it, sometimes mixing in other supplements or treats.