I agree with Kraken as well, but I would also add that it can vary for you from dive to dive.
Here's an example, I was in Dominican Republic in February for a couple of weeks. In the beginning, I was diving wearing a 1mm suit and completely comfortable. After about a week, I had become pretty used to the 80 degree water and eventually had to add a 5mm jacket and I was getting cold on the second dive of the day even with that by the end of the two weeks.
I can dive to the upper 40s in a 7mm with the jacket normally, but if I've been away (from my home in Maine) diving in warm water, when I come back, I need to dive in my dry suit.
So, while it varies from diver to diver, it also varies for each diver from dive to dive. The one thing to remember is it is better to err on the side of being a little too warm rather than too cold, especially in colder water.
Jeff