It lasts pretty much forever. Bearings are floating on oil so as long as it's never run dry or doesn't rust and seize internally. Over a very long period of time the wheels will develop some wobble from wear but we're talking hundreds of thousands of miles. If there's some debris that passes through engine it'll touch the housing and self destruct instantly.
Then there's always a few defective ones which just fail early, nature of manufacturing.
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