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There isn't an answer to that other than to test your water just now for pH, ammonia, nitrite, nitrate, phosphate, silicate, calcium etc etc and then test the exact same next week and every week after... ecentually you will learn when your tank needs a WC
Each tank is differerent and depends on your filter, photoperiod, skimmer, live rock, live stock etc etc
whats the dimensions on your tank? a 72x24x24 with a 60 gal sump? I used to do monthly WCs on a 200 gal tank...
To be honest, they are probably not all that that important if your other parameters you do check for are okay... as i guide i normally say a monthly 10 - 25% water change depending on how quickly your nitrate creeps up and your pH creeps down
but in a 240 gal tank you may get away with a lot more than a bimac in say, a 40 or 50 gal tank