The length of a mortise tenon varies depending on the specific woodworking project and the strength required for the joint. In general, the length of a mortise tenon is typically about 5 to 2 times the thickness of the projecting member or rail that receives it. This longer length helps to provide stability and prevent the tenon from pulling out of its mortise when subjected to stress or weight. The precise length may also be influenced by factors such as the type of wood used, desired aesthetics, or structural requirements.