Finance and Business How long should a pair of undies last? How long should a pair of undies last? All things considered, we'd recommend replacing your underwear every six months to a year. Additionally, keep your eyes peeled for any signs of damage and then replace on your own basis. Related Posts:What color underwear is for love?What kind of underwear do you wear with leggings?Is underwear a correct word?What kind of underwear do British men wear?