It is difficult to estimate the quantity needed to make a mask, because there can be a lot of variables. Of course, the main variable will be how thick the mask is, both on average, and in the thickest parts (for example cheeks bones or jawline if they are exaggerated). In some cases a trial kit should be enough. Here is one example that uses the contents of one trial unit of Dragon Skin.
Ultimately, a trial unit of Dragon Skin 10 contains 2 lbs of material with a volumetric yield of 25.8 cubic inches per pound. This gives you a total of 51.6 cubic inches of Dragon Skin. As every mask is different, it could require somewhat more or somewhat less material to make the piece.