I am a cold water diver. I don’t like diving below 50 in a 7mm, though I know people who do the frozen fin dive on Jan 1st in a 7mm, and water is in the 30s by then. Personally, I don’t like being cold.
If you’ve never done it before and want to try it, I would suggest you plan your dive for no more than 10 min. If you get cold before then, thumb the dive and get out and get warm. Don’t wait until you start shivering.