How Long?

I would actually use both, what canister did you end up getting ?
 
Since the new filter is more powerful the bacteria in it will grow very fast. You might even be able to get away with only waiting 3-4 days.

Personally if the noise bothered me that much I'd yank the old one right now and just do water changes if the NH3 and NO2 get out of spec.
 
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