Depends on what you have or want to put in the tank, whether you are wanting to use this as a form of filtration, and what you think looks good.
If you want to plant in the substrate you will need at least 1" of substrate.
If you have burrowing species, you want this to be 4" or more.
Generally with depths of over 2" you need to be careful of anaerobic bacteria building up in pockets where anaerobic breakdown can occur, the end products of this are methane, carbon dioxide, and hydrogen sulfide, which can be harmful to fish. With some types of sand with particularly small particles, this may occur at lower depths.
Some people actually use deep sand beds (at least 4-6") as a form of filtration, populated with bacteria, algae, and burrowing marine organisms. These are designed to cultivate anaerobic bacteria which converts nitrate to nitrogen gas.
I have a fancy goldfish tank with around 1" of sand, I find this works well for my plants and is not deep enough for toxic gas to form. Just to be safe I stir up the substrate once a week whilst vacuuming.
For calculating what weight of sand is required to achieve a specific depth for a given tank there are a number of calculators online. One is available at ultimatereef.net:
The amount needed will be specific to your tank. Two tanks with the same water capacity will not necessarily have the same dimensions, so the surface area of the bottom (which you wish to cover with sand) will vary.