I use Chavant NSP Plasticine clay (hard). As it stands right now, using a pepped helmet as my armature/skeleton, I've used up almost 4 pounds. By the time all is said and done, I'll likely have used 5-6 pounds worth.
As a note, when working with harder clays, its nice to have a heat gun around to heat it up prior to application which makes it much easier to get into the rough shape you want, rather than straining the heck out of your hands trying to force it into the shape you want (prior to scraping and whatnot). Any method you can use to heat the clay up to soften it will do just fine, I just like the heat gun because its cheap and it can stay immediately by my side as I work.