The amount of liverock will vary due to the density of the rock and personal opinions.  Denser rock will not fill in as much of an area as less dense rock, so you may need more dense rock for the same sized aquarium as less dense rock.  For example 5lbs of a dense rock may only be a piece of rock the size of a soft ball, but a less dense rock may be the size of a basketball.  But most people will try to use between 1/2 to 2 pounds of rock per gallon of tank water.  So for a 120 gallon tank you would want between 60 and 240 pounds of rock.  I personally usually go with around 1 pound per gallon, so I would recommend around 120-150 lbs of rock.

For live sand it will depend on what you are trying to acomplish.  If you are looking at having a deep sand bed (DSB), then you would want about a 4-6" deep sand bed in your tank.  If you just want it there for the look, you could have anywhere from 1/2" to 3" of sand in the tank.  Assuming your tank is a standard 120 gallon tank (60x18) you would need 230 to 340 lbs to get a 4-6" sandbed.  To get a 2" sand bed you would need around 120 lbs.