My small miniature poodle, Potsie(12.5 lb.), has grade 4 luxating patellas in both hind knees. I give him 1/2 a tablet of Cosequin DS plus MSM, along with a fish oil capsule daily. The directions on the bottle of Cosequin recommend daily for about 3 weeks, then every other day, but due to the severity of Potsie's case, I give it to him daily, with my vet's blessing. Potsie is amazingly not lame, even though his knees are always out of place (even if put back in, they immediately slip back out). He does have the bow-legged appearance in the back (common to this condition), but doesn't skip or limp. My vet is beyond amazed!
It is KEY to keep such dogs on the slim side, as extra weight puts even more stress on the joints. Fortunately, Potsie isn't much on excess jumping, which is good.