I buy 3 yards when i don't have a project...if knit, it will make 2 prs of pants, 2 skirts, 1 skirt and 1 top or 1 pant and top, 1 jacket or 1 bog coat (my fave), if quilting cotton, it will make queen size borders without a seam (with corner blocks) or a background fabric for any size quilt.
If on sale for a really good price, then i buy 6 yards and cut borders to put away (up to 6 inches x 4 strips leaves 20 inches x 6 yards for the stash) I cut the borders first so that i know they won't get shortened while working on something else. i have a box with pre-cut borders in 2s, 3s, 4s, and 6s... it's not that i don't use borders in other widths, but i know these work with a lot of my normal block preferences.