You may be confused by the many guidelines that there are for how long milk can be safely stored. I see red every time I read one of the "helpful" breastfeeding guides distributed by the formula companies that say things like "breastmilk can be stored for 24 hours, if breastmilk is left at room temperature you need to throw it out". Of course their guidelines are short - the more breastmilk you throw away, the more formula they sell. The following guidelines are from La Leche League, an organization that pioneered breastfeeding advocacy when there was no such thing. These guidelines are based, as much as possible, on laboratory research testing breastmilk at different time-points for bacterial contamination.

Storing milk in 2-4 ounce amounts may reduce waste. Refrigerated milk has more anti-infective properties than frozen milk. Cool milk in the refrigerator before adding to frozen milk to prevent thawing.

Is my milk spoiled? Sometimes milk that has been stored develops a soapy or metallic taste. This does not mean the milk is spoiled. It is because the enzymes that help digest breastmilk can sometimes break down some of the fats, resulting in what adults taste as an "off" flavor. If your baby drinks the milk, you don't need to do anything different. If your baby refuses this milk, scalding the milk before you store it will stop the activity of these enzymes.
Breastmilk that is actually spoiled is pretty unmistakable - it will smell positively foul. Don't worry, you'll know.

Human milk can be stored:

• at room temperature (66-72°F, 19-22°C) for up to 10 hours
• in a refrigerator (32-39°F, 0-4°C) for up to 8 days
  Store in the back of the refrigerator, not in the door.
• in a freezer compartment inside a refrigerator (variable temperature due to the door opening frequently) for up to 2 weeks
• in a freezer compartment with a separate door (variable temperature due to the door opening frequently) for up to 3 to 4 months.
• in a separate deep freeze (0°F, -19°C) for up to 6 months or longer.
• Talk to your doctor about more conservative storage guidelines if your baby is premature, immune compromised, or seriously ill.

Containers: Refrigerated or frozen milk may be stored in:

• Hard-sided plastic or glass containers with well-fitting tops
• Freezer milk bags that are designed for storing human milk
• Disposable bottle liners are not recommended, unless they are specifically designed for breastmilk storage.

Warming

• Thaw and/or heat under warm, running water.
• Do not bring temperature of milk to boiling point.
• Gently swirl milk before testing the temperature. Swirling will also redistribute the cream into the milk. (It is normal for stored milk to separate into a cream and milk layer.)
• Do not use a microwave oven to heat human milk. Microwaving kills many of the living immune cells that fight disease. Microwaving any beverage can also create hot spots that can burn your baby's mouth.

Can you re-use a previously heated bottle of breastmilk? I think you can, but check here to see the evidence.

Thawed (previously frozen) Milk
If milk has been frozen and thawed, it can be refrigerated for up to 24 hours for later use. It should not be refrozen. It is not known whether milk that is left in the bottle after a feeding can be safely kept until the next feeding or if it should be discarded.

According to THE BREASTFEEDING ANSWER BOOK, page 228, research indicates that human milk has previously unrecognized properties that protect it from bacterial contamination. One study, Barger and Bull 1987, found that there was no statistically significant difference between the bacterial levels of milk stored for 10 hours at room temperature and milk that had been refrigerated for 10 hours. Another study, Pardou 1994, found that after 8 days of refrigeration some of the milk actually had lower bacterial levels than it did on the day it was expressed.

Expressed milk can be kept in a common refrigerator at the workplace or in a day care center. The US Centers for Disease Control and the US Occupational Safety and Health Administration agree that human milk is not among the body fluids that require special handling or storage in a separate container.