you can use as little or as much as you want. keep in mind that the deeper the gravel, the more room for crap to get trapped down in it, and the longer it will take to clean every week. plus, the more gravel you have, the less water will be in the tank.
Not only the area of the tank - but also the density of the gravel's material. For example, if you use something like Soilmaster/Turface/Aquatic Soil as your substrate you will need far less weight, as these are extremely light products.