Apple Pie-Filling Cake

Ingredients:
Cake -
1 white cake mix
1/3 cup oil
2 eggs
1/2 cup water
1 can (21 ounces) apple pie filling

Glaze -
1 tablespoon margarine, melted
1 cup powdered sugar
1/2 teaspoon vanilla extract
hot water

Directions:
Preheat oven to 350 degrees.
Mix together cake mix, oil, eggs, and water.  Pour mixture into a greased 9x13-inch pan.  Marble in apple pie filling.
Bake 30-35 minutes.
Combine margarine, powdered sugar, vanilla, and hot water.  Drizzle glaze over cake.

??? -- how much hot water?