There is no simple answer to this. It depends on the ingredients, the temperature and how long it is kept warm for amongst other things.
My Guy Fawkes mulled wine is basically 2 litres of good red warmed through with bits of peach, cinnamon and cloves. Keep the lid on and do not boil. When ready to serve add a good wallop of brandy and port and enjoy.
Warn drivers about the contents or just don't give it to them.
Many years ago I remember having to carry my almost teetotal mother-in-law to her car (she wasn't diriving) because she had enjoyed it too much.
If you son't boil it and keep the lid on it will still be fairly alcoholic.
Well actually, Gef, I think you've provided the simple answer: if the wine doesn't get hot enough to vaporise and drive off the alcohol, it will still be alcoholic. Generally recipes like stews and casseroles (and of course flamb� dishes) that include wine or spirits are cooked at a sufficiently high temperature to drive the alcohol off, leaving the flavour but not the intoxicant, while mulled wine is usually warmed rather than boiled, and therefore the alcohol remains.