When making your own curtains, or even when buying them 'ready made,' you need twice the length in fabric as the length of the windows you are covering. So, if she needs 100 yards of fabric, that would mean you have 50 yards of window to cover. (No, I've never made curtains, but I've had them made for me; and I've bought plenty; and my mom's made them.) Okay, thinking further ... long windows? ... then how many yards of fabric she would need would also depend on the width of the fabric, as she'd be making panels and sewing them together.