You will get a difference of opinion but the research that I read and made sense as it was done by a university and seemed very thorough came to the conclusion that 16 hours was the optimum time, any longer made no difference. I do mine for 12- 16 hours then rinse well, it means my pony can have more time eating and has been laminitis free, back in work and best of all reasonably satisfied with the amount he gets to eat, while remaining slim.
Holy heck! 12 hours!! Does that remove all calorific value? I want to cut down, not completely remove as he gets very little else being on box rest unless I walk him inhand for a mouthful of grass. Teeny amount of speedibeet to disguise supplements.
It will remove most of the calories, this will make it safe to feed more to the horse, top up lost vits and mins with a supplement that has no additional calories. If he is on box rest he will not need many calories unless you want to put weight on.