Nothing lasts forever. That said, many items are less obvious than the spoiled milk in your fridge; there's no expiration date or other obvious signs they have outlived their usefulness.
So how long should you let your stuff linger before replacing it? We talked to the experts and nailed down a "best by" date for a number of common household items.
Air filtersReplace after: six months.
Why: Filters become clogged with dirt and other particles, reducing the energy efficiency of your heating and cooling systems, says Rozanne Weissman, a spokeswoman for the Alliance to Save Energy.
ComputersReplace after: four years.
Why: Unless you're using your computer for very basic functions (i.e., word processing), technology will have likely advanced enough that you need a significant upgrade, says David Carnoy, the executive editor of CNET, a technology review Web site. Rather than trying to put a new processor into an old computer, you're typically better off buying new.
"With laptops, there isn't much you can do to upgrade them," he says. Upgrading your desktop is possible, but unless you're a technology whiz, that process can be as expensive as buying a new computer (and infinitely more complicated).
CosmeticsReplace after: three months to two years.
Why: Microbe growth in makeup can cause skin irritations and infections, explains Paula Begoun, the author of "Don't Go to the Cosmetics Counter Without Me." The worst offenders are liquid cosmetics, which build up bacteria more quickly. Keep mascara for a maximum of three months; other liquids and creams (eyeliner, foundation) for no more than a year. Powders (eye shadow, blush) can be used safely for up to two years.
Fire extinguishersReplace after: 10 to 12 years.
Why: The pressurized contents of a fire extinguisher depressurize over time. "Eventually, it just becomes a block of 'stuff' inside the cylinder," rendering it useless in fighting fires, says Tom Olshanski, the director of external affairs for the U.S. Fire Administration.
Extinguishers can deteriorate faster if left in a high-humidity environment. Check your extinguisher on a monthly basis for corrosion, a sure sign air is leaking out, Olshanski suggests.
MattressesReplace after: seven years.
Why: Mattresses wear out over time, says Kathleen Huddy, the textiles director for the Good Housekeeping Institute. One that sags or is uneven won't keep your spine in line as you sleep, which means you may have trouble sleeping. It's likely you'll also experience neck and back pain while awake. There's also an ick factor to old mattresses, she explains; they collect dust, dirt and allergens that can't be removed.
PerfumeReplace after: two years.
Why: Exposure to light, heat and air will lighten and alter perfume notes, explains Andrea Walker, a makeup expert with cosmetics company Sephora. "It weakens the scent," she says.
PillowsReplace after: two years.
Why: Pressure on the pillow as you sleep causes it to flatten and develop uncomfortable bumps and lumps, says GHI's Huddy. To test if your pillow still has life in it, she says, fold it in half and rest your hand on top for 30 seconds. Then let go. "If the pillow pops back, it's fine," she says. If it remains folded, it's time to buy a new one.
Running (and walking) shoesReplace after: 300 to 500 miles.
Why: Foamlike material in the midsole of the shoe stops bouncing back. "You lose the shock-absorbing capacity," says Dr. Jim Christina, the director of scientific affairs for the American Podiatric Medical Association. And that can lead to heel and arch pain, or even stress fractures. Judge how much life is left in your shoes by the way they feel.
When a running shoe becomes uncomfortable, it's time to toss it. You can lengthen the life of your running shoes, says Christina, by alternating between two pairs. That gives the foam midsole time to decompress between runs.
Smoke alarmsReplace after: 10 years.
Why: Constant stress on a smoke detector's sensors from particles in the air -- everything from cigarette smoke to pet dander to pollen -- render it unreliable, says the U.S. Fire Administration's Olshanski. "Its ability to sense smoke deteriorates," he says. The result will be one of two extremes: Either the smoke detector will sound the alarm for just about anything, or it won't go off at all.
SpicesReplace after: one year.
Why: Dried herbs and spices lose their flavor over time, says Tina Ujlaki, the executive food editor for Food & Wine magazine. Whole spices, such as star anise or coriander, may last slightly longer because less of the spice is exposed to the air. To tell if your spices are past their prime, open the bag or jar and take a whiff. "If there's no scent, there won't be any flavor," Ujlaki says.
ToothbrushesReplace after: three months.
Why: Toothbrush bristles frayed from use remove plaque less effectively than those of a new brush, according to the American Dental Association. Swapping out your brush will also limit exposure to bacteria that build up on the bristles.
This article was reported and written by Kelli B. Grant for SmartMoney.
Published March 23, 2007