Ah I have been doing one every other day, as didn't want to overdo it, hes not ridden, weighs around 500kg. Will try and speak to vet Monday, we keep missing each other at the moment !
Friends horse was on 1/2 sachet in morning feed for about 6-8 years. He is a 17.2 WB type weighing in about 700KG and owner has only recently upped it to 1 sachet per day due to arthritis in the neck. Depends on how horse metabolises it and what degree of arthritis if any is present. I know of another horse who is on 2 sachets per day and hasn't nearly as many problems as friends horse in terms of arthritis. All horses are different, sometimes you have to play around with dosage to get the required dose right.