We leave them on for the whole match. Lots of people were checking them out at the Ozark 3 gun. Saw lots of use for them on a few stages, 7 and 5 come to mind.
Slammer, I used the K9 at the bunker stage while shooting from the tree as well.
For 556 guns its great, with 762 NATO guns (what I shot at the match) it will dislodge the teeth with every shot, however the k9 still provides a solid anchor before the shot is broke. For 556 guns, the K9 stays put.