I've given glucosamine to my dogs for years, and I can clearly notice a difference in their joint health and how stiff and sore they are after long runs ('long-run': 15-25 miles, measured on GPS collars). When I run out of glucosamine, or just drop off of giving it to them for whatever reason, I see them fall back pretty quickly. And when I restart them on it, I notice a response in a few days.

I also give give them fish oil pills, but I've never been able to notice a difference.