You usually don't think about the core plugs until one of them might fail, and then you get really upset. Core plugs may occasionally pop out when nearly new if they were not properly installed. Even professional engine rebuilders sometimes get it wrong. But if the plug stays in through the first few heat cycles it is usually in there forever. After that the failure mode is rusting through the plug. If you find a pin hole in the plug, that means it is rusted through and has to be replaced. If one is rusted through, the others are likely not far behind, so it is a good idea to replace them all at the same time.

These plugs are always replaced during an engine rebuild when the block is hot tanked for machining. If you keep antifreeze in it for anti-corrosion the plugs should last until the engine wears out. If you have plain water in the cooling system, then the core plugs are likely to rust through from the inside after several years. If they are rusted somewhat thin to begin with they might rust through while sitting in storage with no fluid in the engine.