Becky --
I've used it before. I think you are only to give it 1-2x a day (I don't recall which -- I'm thinking horses metabolize it slower than people, though).
You can buy buffered aspirin gel / paste at the feed store, TSC, or online, however, I found it near to impossible to dose it down to mini size. But, if you look at those products (the details online), I think it will give you guidance as to dosage, how often, and how long you may continue it for the horse.
What I used was simply Buffrin. I think that is more buffered than the generic buffered aspirin, but you could compare the buffering ingredient and amount to a generic (where i was just didn't have a buffrin in generic form). I put the tablets in a slice of apple.
What I'd used it for was a horse who showed signs of laminitis. We put him on basically stall rest (fat pony) and gave him this aspirin 2x a day, I think. I had read it helps circulation and is ideal for the problem we have. He got better fairly fast (it may have not been laminitis, though) and we put him and many of ours on a low carb feed and no problems since.
Good luck,
Jill