Depends on several factors:
The size of the fish in question
The types of fish (your average clown, for example, can be fed once a day, whereas your average anthias needs food 4x or more daily; herbivores may require more raw volume of food simply because so much of it is roughage)
If you have any inverts that need supplemental feeding
What kind of food you're using (some are much richer nutritionally than others, there are different particle sizes, different ratios of food vs liquid, cube sizes, etc, AND different species will need different foods)
...did I miss anything?