Freshly Pumped Breast Milk (never in a fridge or freezer) can be left out at room temp (66-78 degrees F) for 4-6 hours.

Breast Milk stored in a Refrigerator (less than 39 degrees F) can last 3-8 days.

Breast milk store in a standard freezer (0-4 degrees F) will last 3-6 months (like a top-bottom or side by side fridge & freezer).

Breast milk stored in a deep freezer (less than 0 degrees F) will last up to 12 months.

Remember to rotate your stock and always check your breast milk before giving it to your baby - if it smells or tastes funny then its likely gone bad. 

When you refrigerate your breast milk or freeze it and then let it thaw back out, you'll notice a white residue that often sticks to the sides of your bottles or milk storage bag.  That's the fat in your breast milk, it's perfectly normal for breast milk to separate when cooled.

When you give the bottle to your baby, be sure to gently swirl the bottle around to mix the important fat back into the milk.  Do not vigorously shake the milk to mix it together as some research says this breaks down some of the milk proteins.

Yes, if you give your baby a bottle of breast milk and they don't finish the bottle, you may leave the bottle out and offer it again within 2 hours.   Some may argue this is just a rule for formula and not breast milk, we recommend throwing the bottle out after 2 hours because bacteria from saliva can for in the nipple of the bottle.

If the bottle has been heated or warmed, you need to use the bottle or toss the unused amount after 2 hours.  Do NOT put the bottle back in the refrigerator.

It's important to keep breast milk in the right temperature range, leave it in your diaper bag on a hot summer day and you risk the milk going bad if it's not drank in time.  If you're going for more than 2 hours or so you have to keep the bottle cool, but then if you're bottle is kept cool - how do you warm it up while you are out?

• Pump in the Car.  I had a car adapter for my pump and was always pumping on the go.  It also came in handy when we went to family outings in the park or long day trips.  I pumped in the car right before arriving or excused myself to go pump on those day-long outings.  When I finished pumping I made a quick bottle and was ready for the next feeding because fresh milk can last up to 8 hours (if you keep it out of the sun).
  
  
• Bottle Coolers.  If you're going to be gone for several hours and need to keep your milk cool, you'll find LOTS of different coolers made especially for bottles.  I liked the Munchkin Cool Wrap Bottle Bag because it kept the bottles cool for our longer outings.  It claims 13 hours, however I'd say it kept cool for more like 8 hours while riding in a stroller on a 80 degree day. The Munchkin Bottle Bag operates with ice packs that you slip in; I stored my ice packs in the freezer so I was always ready to go.  Other coolers require you to put the entire cooler in the freezer - this may or may not convenient for you.  Also, the Munchkin bag claims to hold 3 standard bottles, I used Avent and could fit two of their bottles in the bag.
  
  
• Ask for Warm Water.  If you are quick running out the door and toss a bottle in a cooler, chances are the bottle will still be cold when you go to serve it.  There are lots of ways to warm a bottle up... warm it in your hands or between your legs, take it out 15-20 minutes before you think you need it, even try serving it chilled (some babies don't mind).  My baby always wanted a warm bottle, so I often would find a nearby Starbucks (why not treat myself too) and request a 1/2 glass of warm/hot water (just about every fast food place will do it too).  I'd drop the bottle in for a minute or less and we would be warmed up and ready to eat. Be sure to ask for 1/2 a glass so you do not overflow the cup when you put the bottle in!
  
  
• Bottle Warmers.  For the on-the-go mom, you can also purchase travel bottle warmers.  They make ones with car adapters and ones with warming packs that you can activate, boil, etc. before putting the bottle in to warm.  Personally I haven't tried many travel warmers, but do know some moms that rave about them.  If you want to save some money, you can warm the bottle in advance and put it in a coolie sleeve (they make tall ones for beer bottles) and it will keep the breast milk fairly warm.  If you do warm the milk be sure to use the bottle within 2 hours.

No breast pumping Mom ever wants to throw out their "liquid gold," which begs the question... How Long Can Breast Milk Sit Out?  A lot of the answer depends on what happened to the breast milk prior to sitting out. 

Freshly pumped breast milk will usually last about 6-8 hours at room temperature.  This is assuming you don't have lipase issues which break down your milk faster than normal.

Freshly pumped breast milk stored in a cooler with an ice pack is good for about 12 hours before it needs to be refrigerated or giving to your baby.  This is a very loose estimate since cooler and ice pack temps can vary greatly, but it gives you a good idea for instances like pumping at work where you do not have a refrigerator on hand.

Breast milk that has been frozen and then thawed has a MUCH shorter lifespan since some of the breast milk is broken down in the freezing process.  It is best to defrost your breast milk slowly in the refrigerator, which takes about 12 hours.  Once the breast milk is completely thawed it should be used within 24 hours.

Thawed breast milk has a lifespan of about 2 hours at room temperature, after 2 hours it should be discarded.

Once heated (in a glass of warm water or a bottle warmer, never in the microwave) breast milk should be given within the two hour time frame or be discarded as well. Regardless if previously refrigerated or thawed.

Once a bottle has been partially used, it is best to only offer the same bottle within the original two hour time frame that the breast milk was heated or served. Much controversy exists here between LCs, Moms, Experts, etc. because little research exists at the moment.  To avoid bacteria risks we choose to recommend staying within the two hour time frame.

All this being said, it's still best if you plan to freeze the milk to do so within 24 hours and if you're not planning on using fresh milk to refrigerate it as soon as possible.

There is little research on expressed milk and thawing/use, so please remember these are just guidelines.  You milk should not taste sour, most of the time it has a sweeter taste/smell to it.  Although, some mothers have reported a soapy smell/taste once thawed, which is thought to be safe as long as baby will drink it. Also, please note these these guidelines are for full-term healthy babies. 

This really depends on your pumping plan but here's a few tips & suggestions...

   ~ Keep a day's worth of milk in the fridge and maybe an extra bottle for safety and you can freeze the rest if you're working on building up a "stash of liquid gold"

   ~  Milk is good for up to 3 months in a regular freezer or even longer in a deep freezer.

   ~ Remember thawed milk should be drank in about 24 hours.

   ~ I always took enough milk for the next day out of the freezer the evening before and put it in the fridge. Then in the morning the milk was almost completely defrosted so I made up our bottles for the day at one time.

   ~ If you're planning on quitting pumping and just going to frozen milk plan for about 1,000 ounces a month to be consumed.  This gives you 25-30 oz a day for your baby, plus a little extra for growth spurts, wasted bottles, and mixing into food.

   ~ If you're freezing most of your milk remember to start rotating your stash by using at least some of the milk in the freezer so it doesn't go bad.