This will vary based on the substrate, the tone of the artwork, and the speed and power settings used. For example, an image that is 100% black will engrave deeper than one that is 80% grey, even if the same speed and power settings are used. The same graphic will engrave deeper if the operator reduces the speed or increases the power at which it is engraved.
Some materials are ideal for producing depth, such as wood. Other materials, such as fabric, are too delicate to achieve much depth. Since there is not much volume there to begin with, this material requires just a quick surface etching.