This is a really hard question to answer easily. Sometimes you want a very light dusting of glitter to give an effect and sometimes you want it heavily covered with no background showing through.
From our test of Regular Chunky Flake (0.040) glitter, 1oz will completely cover 1 square foot.
Painting the background of what you are covering first really helps. For example, if you are covering a surface with silver glitter you can paint the surface a light gray. That way if your glitter does not cover every square inch it will not be noticed.
See GlitterUniversity.com for more information. Craft Coverage and Poly Coverage.