I have dove under ice, in my 5mm Elios Low Density Heiwa, on a New Year's day. I spent 3 hours swimming around and diving. My hands and feet froze by the end, which is why I had to get out. The water temperature was 4C degrees, and the air temperature was 0 or so. After the first 45 minutes I had also flushed the inside of my suit, from waist down, with water. I was warm throughout the whole dive, except at the moment of flushing of course.
If I had been doing line diving, where one rests at the surface for lengthy periods of time, and does not stay active much, then certainly I would have lasted maybe only an hour or so. But that at least gives you an idea of how effective these suits are.
In the end, it will be your feet and hands, that determine the need to get out of the water, if you want to avoid damaging them.
For the average person, usually a 7mm for less than 12C degrees, a 5mm for 12-17C degrees, and a 3mm above that to 23Cdegrees. Maybe adjust for women to 7mm-15C, 5mm-20C and 3mm-26C.
Reallly depends on your activity as well. If you are active, then you get away with colder temps.