It's all relative to your needs, it's not a set answer.
If you have a FOWLER (fish only with live rock), you might be able to get away with fewer changes, if you have a mixed reef then you might need to keep up with more regular changes.
If you have a primarily zoa only tank, then you might need fewer changes compared to an sps dominated tank.
Although a general rule to start with would be around 10% bi-weekly, or 20% monthly.