Chain wear is almost totally dependant on riding conditions and terrain. It is something that you can change often and thus avoid excess wear on cassettes and chainrings.
Wear is defined as chain "stretch" 1/16" in 12 full links (12") is worn to the point of repair. Stretch really isn't streatch but rather the wearing away of the inside of the rollers which causes the taught chain to be longer when measured.
Wet riding is hard on the chain (use a wet lube with persistance). Dry riding can be death to a chain also as the dust that accumulates is like grinding powder if it gets inside.
Anything that keeps wet and dirt out of the inside of the chain is a good thing.
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