It depends on your set-up. I currently use an 82 Qt rectangular cooler and a 12-inch Bazooka Braid, which is probably not as finely-meshed as a "Denny Braid", so I notice that it takes me about 2 to 6 Qts of Vorlaufing to get it as clear as I'd like before it goes into the kettle. It usually starts out looking like an opaque milky-brown colored smoothie and then transitions to a translucent, watery, slightly more darkly-colored fluid (wort). When I pour the vorlauf back into the cooler I check to see whether there are any fine sand-like particles in the grant (a cheap 2 Quart white plastic container I use for vorlaufing). If there are, I keep vorlaufing. If there aren't, then we're good to start running off the wort into the boil kettle.
I don't like the idea of extracting tannic bitterness from grain husks. Since I go to great pains to ensure that the mash pH and temperature are optimal (e.g., pH of 5.4 and T<170F), I don't wish to undo those efforts by allowing a bunch of grain husk material into the boil kettle during sparging/lautering. That's just me.