This routine from Robert Zipprich will return the total length of any number of lines, arcs, and/or polylines that you select. Load the LISP file and type in the command AddLine. Select the desired entities, then press Enter. The routine will tell you the total length of the supported entities and the length of each individual entity, both at the precision defined in the Units command. It will also give you a notice regarding any entities that were filtered out. Robert's code is also a good start if you want to add the ability to include other entity types such as Mline, Spline, or even Circle objects.