there is no definitive answer. Anywhere from 8lbs - 2lbs.
The actual manufacturer of the suit makes a huge difference as the fabric composite can differ, the age of the suit makes a difference, the diver's weight and body type make a difference. There is no way to tell how much difference without doing a weight check wearing the suit. Sorry!
As an example - I went from a 7mm 2piece suit to a 1piece a few years ago. I used 16-18 with the 2 piece, I now use 12-14 with the 1 piece. Difference in suit manufacturers: the 1 piece is much nicer, high quality and better made than my old 2 piece, plus it is new so neoprene is still springy and hasn't had any compression (my old suit was 5 yrs old and 100+ dives). The new one is much warmer to me too as it is better quality and no compression of the neoprene. So even though it is less neoprene, I didn't drop alot of weight between the 2 suits.