Changing plugs is often is a ten minute fix for a tuning problem. I change maybe one plug per year for a fleet of 16 airplanes.

Plugs should be changed for the following reasons:
1. The element is burned out.
2. The coil is distorted.
3. Foreign material is on the element.
4. To experiment with a different heat range.

Trash will enter the intake will take a plug out.
Trash in the fuel system will result in a temporary lean condition burning out the element.
Some oils used for after run will contaminate the glow element.
Otherwise problems are tuning and not plug related.

Bill