Bob, before you pull the coils out of the glow plug body, heat the plug coil red hot with a glow driver, and let it cool. That seems to make the coil less prone to break and easier to pull out of the body. I've been doing that for several years since I started flying jets, and it seems to make the plugs last longer, too.

BTW, I haven't discovered any average/normal life span on the plugs. They last varying amounts of time, so it is always a good idea to have spares handy. Your getting 100 + starts seems to be a very long time according to my experience.

Good luck, Les