Everyone knows that you should change the oil in your car every three months or 3,000 miles. But many of us are mindless when it comes to basic health maintenance. Quick: How often should you change your sheets or throw away a kitchen sponge? Read on for some eye-opening answers.  Toss toothbrushes. You should replace your toothbrush every three months, says Matthew Messina, D.D.S., a dentist in Cleveland "Change it sooner if you get a cold or the flu, since toothbrushes can harbor bacteria." Another addendum to the three-month rule: If the bristles become frayed, toss…