For wrapped boxes
A cross wrap needs one loop around the width of the box, one loop around the length of the box, plus ribbon for tails and the bow. Measure around the box instead of only across the top, because height matters.
For bows
Simple flat bows can use 12 to 18 inches. Fuller gift bows often need 24 to 36 inches. Wired ribbon needs extra length because shaped loops look better when they are not pulled tight.
For wreaths
Plan ribbon in sections. A small accent bow may need 2 to 3 feet, while a wrapped wreath can use 3 to 6 yards depending on overlap and wreath diameter.
Cutting tip
Cut one test piece first. If the bow looks cramped, add 8 inches to your batch length before cutting the rest.