ORIGINAL: Deadeye
If you tune your engines properly (meaning slightly rich), your glow plug can last a heck of a long time. I have been flying for 6 years. I have averaged 30 gallons of glow fuel a year on a dozen two and (mostly) four strokes. In that time I have replaced two glow plugs.
Thanks, I was trying to get an idea of what the life of one of those things is and how many spares to carry. Sounds like one is plenty, which is what I was thinking anyway.