Injections have done wonders for my mare who has arthritis in her hocks, although her case is not that severe. Now, I am not familiar with the self-injection procedure and always had my vet come out to do the injections. This was done annually or bi-annually depending on how often the horse was sore... I've heard of the daily/weekly method but I'm just not familiar with it. I feel too inexperienced as you need to inject directly into the joint/affected area for the medicine to work and there is a risk of infection each time you inject. I also never injected glucosamine... Only steroids/corticosteroids, which are longer lasting, hence why I only had the injections done once or twice a year. This is another option you can consider, it's more expensive although probably is just as costly as what you will pay for weekly/monthly injections (I think in my area I pay about $400 per round).
Anyway, I would trust the directions of your vet. What you see on the bottle doesn't take into account case by case basis, and your vet knows your horse best.