It all depends on the way the car is used and maintained, usually at about the 150K mark. Those that develope vacuum leaks and need to replace the lines do it then.
If the engine is always bathed in synthetic, it usually lasts longer. Same is also true for those who rev the engine higher, getting the oil hot. Frequent short trips with the outside temp below 32F cause the separator box to clog.

In short, there is no way to define when the box is getting close to being clogged, or the passage from the block to the box.

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My name is Klaus and I am a V ♂ lv ♂ holic.