I work as a dental assistant and the ones that have to come in every three months are the ones with very poor gum health. We refer the really really bad cases out.  The amount of "stones"/calcified material that I suction out of these guys mouths is disgusting. (We nicknamed them stones cause they look like tiny pebbles. Cause you imagine how long you'd have to not brush/floss for that food to calcify into a lump that big?!)