It's a little like asking, 'What's the best car?'
You've given a clue by stating it is a FOWLR system. Next questions that need answering include: what is the size of the system, all the equipment running on the system, and what is the biological load? Lastly is how advanced the hobbyist is in the area of marine life form husbandry.
Water changes generally fall into one of two categories: Either it is to maintain stability of water quality/constituents; or it is to export undesirable constituents in the water.
Now. . .Assuming the system is 'normal' with all the required equipment, and of about 75 gallons and not overstocked, these are my recommendations:
For stability, a 10% weekly change is good;
For export, a 25% water change every 3 weeks
Most people new to the hobby should use the export recommendation. People with a lot of experience and 'better' equipment can use 25% every month for export.
When there is a known water quality problem, you want to do larger (80% or more) water changes.
When performing the 25% or larger water changes, be sure to follow good practices: How to Make a Successful Water Change.
I hope the above helps. If you want to provide more specific information, I may be able to recommend a more custom water change maintenance.
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