I agree with the above
If you test the tank for ammonia and it is 0 or close you have enough.
If you test the tank water for ammonia and it reads any more that say 0.20, then you may need more biomedia.
Its not about a certain volume of media, its about the proper amount of media that coincides with the ammonia (and other deleterious substances) put out by your particular bio load, and as Tom said, as fish grow you usually need to increase the amount, it is not a static.
It also has to do with your water change schedule, if you constantly do regular water changes that dilute the metabolism by-products that produce toxic substances (which include ammonia) (or chronically toxic substances like nitrate) you may not need as much, than if you were only do a partial water change once every 2 weeks or worse yet, only once per month.