The ratio of eggs to milk in a quiche can vary depending on personal preference and the desired texture. A commonly used ratio is about 1 part milk to 1 part eggs. For example, if you use 1 cup of milk, you would typically use 4 large eggs. However, some recipes may call for a slightly higher ratio of eggs to milk for a denser and richer filling. It's always a good idea to follow the specific recipe instructions to achieve the desired results.