I will answer your question if you can answer mine . How long is a piece of string ?

Point is I think , that the question can not be answered other than theoretically as we are dealing with many unknowns as well as a piece of natural material with no control over conditions of use or treatment . Some stocks seem to last the life of the gun others don't and I have no doubt that some due to the design and fitting are more prone to break than others . May be you should have asked "which stock heads are most prone to breaking "?