Most joint supplements have not been proven to work at any dosage. As well, because they are not regulated, a company does not even have to properly label a container or measure the amount of whatever "supplement" is in the feed.
I don't feed oral supplements because they simply aren't proven to work. Get a vet involved and develop a consistent program for your horse. Based on a simple flexion and lameness exam, a good vet can recommend a plan of action. Many horses need only a round of Adequan, IM glucosamine or Pentosan and possibly a shot of Legend once a year to keep them happy and healthy throughout show season.
The main preventatives for arthritis are plenty of turnout, a healthy balanced diet, keeping the horse a good weight (ie not over weight) and having the horse well conditioned for his job with a varied work schedule.
Good luck! A vet is always your best resource when it comes to your horse's health.
They say money doesn't buy happiness -- well happiness doesn't buy horses!