Just for chuckles, I'm going to make a guess here. Maybe someone will have a hard answer and we'll see how close I am.
I've noticed that if the oil level in my car is right on the one quart low mark when it's cold, it rises to almost, but not quite, a quarter quart when hot. I.E. 4 quarts to 4.2 quarts.
Let's say the temperature difference is equal to ambient to the operating temperature of the engine coolant. I.E. 20C to 100C.
4.2 / 4 = 1.05 I.E. 5% volumetric expansion
100 - 20 = 80C delta T
5% / 80C = .0625%/degree C
So, my rough estimate would be on the order of .06% volume per degree Centigrade.