Well you just said it. Finer grit and aluminum oxide breaks down the slowest.
Actually aluminum oxide rounds, just like ceramic media rounds. SiC fractures to nothing, it is brittle like glass.
Because most ceramic media is 98%+ aluminum oxide. Giant grit, that polishes or takes the finish of whatever abrasive is in the vibe.
But that brittle SiC 60/90 breaks down in no time. SiC 30 broke down in less than 12 hours in my Viking. Maybe 6, I don't know.
That is why vibes are made for finishing and not rounding.
If i were you I would do a half cup sugar and half cup water and add your SiC 60/90 every 3-6 hours and spray wet it as it thickens each time you add.
And do no clean outs till it gets too thick, say after 3-6 additions of grit or as needed.
At least try every 12 hours and do a clean out after 2 adds or as needed to keep your action rolling.
Last rotary I did I added SiC 30 9 times every 4 days. Never did a clean out, just added grit.
When the slurry got too thick I would pour say half of it out and replace it with water.
Different with a vibe, I know that, but see what you can get away with.
You know that a rotary breaks grit down slow and cuts the rocks well over 7 day periods.
How long have you been vibeing those ? But that is deceptive, I put a whole cup of grit each week in a 14 pound rotary.
How much grit will you use in a week changing the grit out every day or every 12 hours.
By the time you have used a cup of grit you may round those more than I did in a rotary....
Anyway, glad you noticed the SiC probably breaking down in short 12 hour periods.