Replacement spark plugs, 20,000 km, iridium spark plugs are 60,000 km. If petrol is good, the battery condition is good and everything is good up to 60,000 it's not problematic.
In General, the recommended spark plug replacement every 20,000 km if the spark plug is not replaced, so there is no maximum engine performance.
If not replaced the engine performance can go down because there's been a buildup of dirt and so on.
There are several factors that cause quick dirty spark plug. A less than perfect combustion could cause the crust.
Dirt was actually a factor outside of the spark plug. For example, burning less gasoline is less than perfect, nice, or filter petrol is less nice, octane gasoline too low. Well it usually raises the crust. There are any remnants he went into the cavity-the spark plug cavities.