I had a run of astringent beers that I traced back to flour in the kettle. But, I'm talking a lot of flour. I had probably a good 2 inches of flour sludge in the bottom of my fermentors.
I generally mash for 15-20min before starting my vorlauf. I vorlauf until the wort is pretty clear, which takes 15-20min. With the wort about an inch deep in the pot, you should be able to see through it pretty well. Not "read a newspaper through it" clear, but you could probably read front page headlines through it. That's for fairly lightly colored beers. Darker beers are harder to gauge.
I have a March pump I hook up to the MLT though, so I can just set it up on a loop, slowly sucking the wort out the bottom and throwing it back in the top. That sets the bed pretty well.
