Did you test right before the water change? The only reason to wait is for the new water to properly disperse... but if you tested right before, then all you'd see is a 10% reduction due to the water change.
In general you can look at the flowrates of your powerheads/return pump to get an "estimate" of how long it should take to turn over all the water in the tank..
So if you have a 100gallon system and pumps/powerheads moving 1000 gallons per hour then a good approximation is 1/10th of an hour or 6 minutes..
I usually just wait an hour or so..
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