@Walter Hinteler,
well if you speak of deep cleaning then never. It is not necessary unless you have issues like gum disease. If you have healthy gums there is no deep cleaning needed.
Yeah our twice a year cleaning includes dental visit meaning looking at teeth and making sure all is good. Considered preventative and is covered in full - I pay zilch.
For the deep cleaning - if it was determined it was needed - is probably covered in part but not all - I think insurance pays 80% of most needed dental care.
Our family has been fortunate as in it has been a while since we had anything beyond preventative care.
Now health insurance - that is a different story....my kids cost me quite a bit.