It depends on what type of setup you plan on having. For plastic plants all you need is enough to hold them down, and an inch or inch and a half should be sufficient. If you go with live plants, you need to look into gravel/substrate closely. If you don't have live plants then you want to keep the gravel clean by regularly vacuumin it to remove the crud that settles down into it. Gravel serves the purpose of increasing the surface area on which nitrifying bacteria live, as well as mimicing the natural environment of some fish. There is no right or wrong amount, so go with what you like.