As @lenadunham once said, women can't have favorite underwear until post menopause (I paraphrased). Here are my inspired reasons as to when and why you should be buying new undies, unless you go commando.
  1. You've had them longer than 2 years
    I'm sure there's some science behind when it's time to get new under garments, but I don't know it or follow it. Unless it's bras. Don't get me started on bra shopping.
  2. They no longer resemble the glorious pair they once were
    The lovely stripes or adorable daisies have faded into grey and off white blobs that are unrecognizable.
  3. Strings and seams are falling off or are non existent
  4. The stains in them have stopped washing out
    It happens to the majority of us. One bad stain a week into wearing them totally doesn't equate tossing them.
  5. The inside cotton is a permanent off white/dingy yellow color
    The farther the color of the cotton lining is from where it started should be an indicator that maybe it's time to toss them.
  6. You have less than a 21 day supply
    YOU SHOULD ALWAYS HAVE A MONTHS WORTH OF UNDIES! Or maybe that's just a college student who doesn't want to pay to do laundry, but seriously. You change them at least once a day. You need more than 14 pairs. And they come in so many cuts for so many occasions. And yes, your 5 pairs for during your period totally count in that 21-31 pairs.
  7. You can't remember the last time you bought new underwear