When do you stop mentioning your dissertation in the cover letter?
My usual advice about cover letters and vitas is to include everything that you can include "with a straight face". A some point you stop putting your high school awards, because it's too embarrassing to mention them; at some later point you stop putting minor honors from your undergraduate studies; etc. 

If you are a recent PhD and are applying for tenure-track positions, you should certainly mention your thesis, unless you have so much other research that it is no longer relevant. 

But you do not necessarily need an entire paragraph. Just now, I looked up two successful cover letters of mine. One had a single sentence about my thesis (for a research post-doc position). One had two sentences, the second of which just said that my work could be suitable for undergraduate research (this letter was for a tenure-track position).  At least in my field, we usually submit a longer "research statement", and so there is little need to go into depth about research in the cover letter.