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Unlike the last generation water pump, the E9X uses an electric pump which means the typical failure point, the mechanical bearing and the impeller, are even more UNPREDICTABLE than the old design.
In the olden days, it is suggested that every other coolant change you swap out the water pump too. When the E36 came along with its self destructing impeller fan, it was suggested that every 60,000 miles you swap those troublesome water pumps before it explodes and take your engine with it. Since then the impellers were improved (gone from flimsy plastic to stainless steel to some sort of lightweight, fancy composite material). With the newer pump design the fan seems to last but the bearings will fail anywhere between 75,000-120,000 miles, so it's back to every other coolant change.
The problem now, with an all electric-driven pump, is that the failure may be other issues, besides the impeller, bearing in the motor (although not mechanically attached to a belt), or the catch-all electrical gremlin. So now you can reasonably expect to replace the pump at ANY interval and it may still fail on you. In fact I've seen several pumps fail on an E9X already. So on an E9X with an electric pump, your best bet is to replace the pump WHEN IT FAILS, not preventively.
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