Eventually. A ratio with less water or milk will definitely harden and without a lot of waiting time. If you are using slightly more water or milk, it will take longer to harden, but will eventually. Heavy cream will take the longest to harden, and often will harden on the outside only if the glaze is on thick enough. If you want to prevent the glaze from hardening, you can add a small touch of melted butter or even corn syrup.