SS, I can't really answer that. There are far too many variables: the size of your quilt blocks, how many blocks you are making, the width of any sash between blocks, the width of the mattress, the size of the overhang. My best advice is to go to your local fabric store or specialty quilt shop and ask them for advice on selecting a block pattern and then using the pattern to calculate the yardage you need.
Just wanted to share this of the final wedding dress (though over a year later!). Thank you so much again for all of your help and editing of the image for me. It was a wonderful day, and a beautiful dress.