I feed it to my boys, 15hh cob and 16.3hh cob and they get 25g a day of it from Feedmark. I think it depends on the quality to how much is needed, I just follow their guidlines.
I fed a heaped dessert spoon twice a day during winter last year and am sure I noticed an improvement in temperament and coat condition. I bought mine online on eBay.
I feed it to our two from March to September to help with SI. Chancer is nearly 15.1 and 570 k and Farra is 16.3 and 720 k - they are heavy hairy horses. I feed a heaped tablespoon twice a day, so I buy a 25k sack from Charnwood Milling which lasts me a year and is the most economical way of feeding it.