You can reuse the media until you notice a large decline in performance.
Another way to tell the media is spent is excessive dust. But since you are
blasting outside & not in a contained cabinet you might not see the dust.
So keep blasting until it works poorly.
"So... I DO have a question for those that have used
aluminum oxide to blast:
How many times did you reuse the media and did it ever
putter out on you?
All I hear is that it can be reused "several" or
"many" times, because it keeps its sharp edge."