SCUBA diving with a dry suit has taught me to respect the water , with a noemal 7mm wetsuit I can manage about 20min in 10deg C water in a dry suit I can manage upto 40min , I think you could last longer that is just my comfort level, Things to remember is to keep some air in the dry suit for two reasons , floatation and warmth. As mentioned before a hoody as well is a good idea