At the moment I'm teaching several Courses in Eastern Canada / North Atlantic. The water temperature is about 7 - 9 Celsius and I am using both the 7mm Aries as well as 7mm W2 with the hooded 5mm vest and the Orion boots and the normal Waterproof 5 finger gloves. We usually get two back to back dives of around 35 - 40 minutes each with a surface interval of around 45 minutes ( give or take ) and the divers are fine. Most of them are female, around 20 - 24 years old and in the 130 pound range ( they are marine biology students at the local university )
Usually they are getting a little bit cold at the end of the second dive, but nothing to uncomfortable.
It think it also depends on your personal comfort level. While I did dives in 4 or 5 Celsius cold water 15 years ago in a 7mm wetsuit of questionable quality, you would never get me in any water colder then 18 Celsius with any kind of wetsuit any more (I know, I'm a whimp...)
Michael