If you don't change your underwear every day , You will face various deseases.The underwear is made from the fabric which is favorable for developing and growing of bacteria.
You should change your underwear daily as your habit.By do this you can prevent below problems.
Strong itching in the genital area
Develop a large number of infections
Urinary tract infections
Rash and sores
You take shower every day.That is the best time for change underwear.But if you don't take bath you should change your underwear everyday.