You may be a bit cold wearing only 6mm in the winter, where local water temperatures typically run in the mid to upper 50's. Body fat is a great insulator, so the more you have, potentially the warmer you'll be. I recently lost about 20 lbs. and it made a big difference in terms of being cold. I'm diving a 9-7-6 Excel "semi-dry" suit which is 9mm in the torso, plus I wear an extra 5mm neoprene shirt/vest under that. I can tolerate 75 minutes in 57 degree water, but not much more. Your best gauge would be to go out in your 6mm suit and see how you feel. Keep in mind too that you may get colder doing multiple dives, and also that as time marches on, the insulation properties of your suit will diminish.
Happy Diving,
Kevin