How many times can I clean and reuse a spark plug when the tip and insulator are still in good shape?

My dad reused his over and over in the DeSoto.

I just cleaned and gapped mine in my old toyota ......... I think this practice may be limited to the copper design plugs. The spark generally jumps from the edge of the copper tip to the side electrode and in copper, it tends to wear those edges quite quickly ..... so in my opinion, for non peak performance applications, it can be done a couple times ......... as long as the tip and the electrode are in decent shape.

spark plugs are $2 each. Just replace them.

High voltage sparks like to jump from a sharp edge , not a rounded surface. Not only do the sharp edges erode from sparking, but the more you clean them, the more rounded the edges get, and the worse they work. Old piston engine planes (like the B-17) used to run 4 prong sparkplugs to ensure smooth running and reduced maintenance.

New plugs are nice and sharp, and explain why a tuneup can result in an even idle.