I find sometimes it takes maybe 30 seconds +/- before I see water flowing through the clear elbow. At first it can be only a trickle.
Depends a bit if the tank is tilted to one side or not. I usually do the initial dump, then turn on the in-tank rinser and let it run 1-15 mins. while breaking camp. Usually does a decent job of rinsing the bottom of the tank - at least what I can see from peering down the toilet.
I would never ever close the dump valve and then run the rinser. All it takes is a distraction some day and forgetting that the rinser is running and you could have a very serious problem.
FWIW, our rinser is visible when looking down the tank. Can you see yours?