Generally, it takes 5-6 yds of material. Amount needed can also depend on whether or not you need covering for the A-pillars or not, the width of the seam, and whether there is a strip along the bottom of the rear window(A-bodies).

AND, if it's mod top material, it can also depend on matching the pattern on the sides of the center section as well.

So it's really not a cut & dried answer, a lot depends on what kind of top and body style.