It depends. The shape and size ofthe room. Furniture, carpet, rugs (to a lesser extent) all play in. If you're having a tough time deciding how much then buy the minimum amount you think you'll need and add and build in until you're satisfied. Trust me you can over-do room treatments and literaly suck the life out of the sound.
Acoustimac.com and GIK.com are two sources you should be familiar with.
O, and my bass traps only go halfway up from the floor as you can see in the avatar.
DJ