What is the substrate and what are you using for filtration?
Also how much ater was changed?
We'll be able to diagnose a lil' better with some more info...
Yah...I've done 75 percent waterchanges in the past and had no issues of cloudy water. The only time that myu water does cloud a little bit is when I clean out the filters and replace the bag of crushed coral, but that's gone relatively quick. Other than that, I get some airation in the water that rises to the top and goes away after like 15 minutes.
more information is needed. The question should not be "how long" it should be "why".
A 150g should not become cloudy for any reason. I could see a small tank with an under gravel filter that has been neglected getting cloudy from just pouring water in...but a 150???