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 the brush. Ragged bristles are less effective and may not get your pearly whites quite so bright, says Dr. Messina.

Mind your mattress. Flip over your mattress once a month to prevent sagging, explains Ronald Rooney, D.C., D.A.C.S., a chiropractor in West Hempstead, New York. "And alternate your flipping between side to side and head to foot." You'll only need a new mattress every 10 years or so, he says. It's time to trade in when you begin waking up with numbness or achiness.

Strip those sheets. It's essential to change your bed linens weekly, says Pamela Georgeson, M.D., indoor allergy specialist with the Grosse Pointe Allergy and Asthma Center in Eastpointe, Michigan. Dust mites—a very common allergen—"congregate in mattresses and pillows and feed off our dead skin cells," she says. The only way to eliminate them is to wash bedding regularly in hot water.

Scrap sponges. Wash your kitchen sponges twice a week in your washing machine's hot cycle, advises Diane Quagliani, R.D., a Chicago-based spokesperson for the American Dietetic Association. Sponges stay damp for a long time, and bacteria thrive at room temperature. "Always throw them into a bleach solution to clean them after wiping up raw meat, poultry, or fish," says Quagliani. And ditch your sponges at the first sign of wear, tears, or stains.

Throw in the towel. Dermatologist Robert K. Ilowite, D.O., in private practice in Hillsborough, New Jersey, says your bath towel shouldn't be used for longer than a week without washing it. "If your perspiration increases in the summer, you might want to change it more frequently," says Ilowite. The higher levels of humidity in warm weather months also contribute to extra bacterial growth in the moist environment of a towel.

Give shoes the boot. It might surprise you that dress shoes worn several times a week might need replacing as frequently as running shoes, says Joseph Caporusso, D.P.M., a McAllen, Texas-based spokesperson for the American Podiatric Medical Association. "Foot pain and overuse syndromes that cause knee and hip pain can result from inadequate shoes." Dr. Caporusso suggests checking the shoes every two to three months for signs of damage, examining the back of the shoe, the insole, and the heel. If any of these are worn or too flexible, head for the nearest shoe repair shop—or shoe store.