I’ve done really a lot of batches (that amount of time with preparation, process and cleaning it absurd )
At the start I’ve probably always tried second run just to see the results. From that experience I came to opinion that amount of kief is highly dependent on variety. When 1st run is done properly, 2nd has only about say average 10-20% yield. I’ve never got anything sensible from 3rd run (probably 1-2% of 1st).
As you said sometimes you can be surprised by the amount even on the second run (these are probably those very frosty varieties), but in general it is time consuming and I resigned on re-mixing used stuff. But sometimes if I don’t have enough material for the run I add some already used just to top it off.
“Yield” can be increased by prolonging the time and mixing “harder”, but it will be greener stuff.