In most cases, it's completely up to you (do you have a skimmer?). You don't really even have to have any substrate (reffered to as bare bottom). You could go with a 1-2 inch sand bed, wich is mostly for looks. You could also go with a deep sand bed, over 3 inches deep, wich boasts denitrification capabilitys (I recomend it if you aren't running a skimmer).
A rough calculation for how much substrate you need is 10 pounds per square foot, per inch of desired depth.