I believe you can go for weeks without changes, providing you test regularly and supplement trace elements as they become depleted.
If you have good nutrient export (macro algaes to consume nitrates/phosphates) and dose as required why do you need to change the water? Simple detritus removal is the only reason I can see. Besides if you have good random flow and no dead spots there shouldnt be a deal of that anyway.
I could test my water now and there would be undetectable nutrients and my alk
would be 9.5 - 9.7, mag 1300 -1340 and cal around 420 and I haven't changed water since new years day. I tend to do 20% once a month.
I first got this idea from watching one of new York steelo's vids and I believe he had gone 80 yes 80 days without a change and I'm sure some of you will be aware of how nice his tanks are.