Black Tank Flush

Until it runs (mostly) clear is my answer. After the tank is fully drained, I close the valve and start the flush. After about 2-3 minutes I open the valve and let the tank drain again. Then I close the valve again and repeat. Each time a few more solids come out. Typically after about 3 times it runs clear enough for me and I turn off the flush. I do have a clear plastic extension on the sewer connection.

Just don't go off and leave it running unattended with the valve closed.