Please provide more context! "Crust" can refer to many things.

For example, are you asking about:

* The Earth's crust? This is the outermost layer of the Earth and its thickness varies significantly.

* The crust of a pie? This can vary depending on the recipe and personal preference.

* The crust of bread? Again, this will vary depending on the type of bread and recipe.

Please clarify your question so I can provide a more accurate answer!