I keep my milk in storage bags. I also put the storage bags in a tubberware in the refrigerator. I was tired and forgot the tubberware full of my breastmilk supply on the counter for an hour and a half, maybe close to two hours out, before I realized and put it back in the refrigerator. The milk still felt cold when I touched it but I need to know how long will they last before it goes bad. There is enough milk for about 15 feedings, so imagine how sad I am right now thinking I can't use it.
Actually that's not the general recommendation for breastmilk. Breastmilk has special properties that prevent it from going bad right away. It can be out for a while before going bad. However, since you won't be using all 15 feedings right away, I'd suggest popping them all into the freezer as the freezer will halt bacteria growth while the refrigerator merely slows it down. Breastmilk that is bad will probably smell and taste bad. Smell it and then taste a drop and you will know!
Here is a good resource for storage handling and times: http://kellymom.com/bf/pumpingmoms/milkstorage/milkstorage/
There are a number of factors that contribute to your ultimate decision to keep or dispose of the milk, but an hour or two on the counter at room temperature will not by itself spoil the milk or make it dangerous or harmful to your baby. There are a number of components in breast milk that inhibit pathogenic activity.
La Leche League International gives 6 hours as an acceptable time at room temperature, with 4 hours being the ideal (http://www.lalecheleague.org/faq/milkstorage.html).
With previously refrigerated milk, I would err on the shorter end of that range, and would consider the age of the milk prior to being left out, but also note that it was cold when left out, and it may not have even reached room temperature during the time it was on the counter.
Personally, I would not keep that much of a supply out of the freezer, but only enough for the next feeding. I know that isn't always possible, but if it is, I recommend it.
What we do with our supply is to freeze expressed milk in bags until needed, at which point it is thawed/warmed in warm water. It takes a few minutes more minutes, but that is mostly just waiting time.
A rule of thumb for storing foods is that every hour storage at room temperature will cause the "best before date" for storage in the fridge (at 7 C or lower) to come forward by one day. This is based on the fact that germs grow exponentially, but at a slower rate at 7 C compared to room temperature. The time for the number of germs to double is approximately a factor 24 smaller at 7 C than it is at room temperature. Food taken out of the fridge will take some time to warm up to room temperature, but note that when you put it back it will also take some time to cool down. So, you can use a wide safety margin of subtracting 2 days from the date you would normally be using the milk, and you should check if the milk is still ok. when you approach that new date.
Answers like this are hard to quantify. Freshly pumped milk last a lot longer than thawed milk does. I am not clear whether this is milk you pumped that day or recently, or milk that had been frozen & thawed.
The fresher the milk, the less likely it is to matter. Room temperature also factors in. Recently here it's been extremely hot & I try to avoid used of air conditioning, so leaving it out in my home mid winter is tremendously different than leaving it out on a day like today that is near body temp & high humidity.
There are guidelines that vary slightly depending on where you check. If the milk is freshly pumped & it's not hot out, I would personally not even consider it an issue. A grouping of bags that are close together will keep one another colder than a bag sitting alone. I would still use them within the original guidelines of whatever I was working with before. That is based on my experience with handling such issues with 3 kiddos & working full time & having mishaps here & there in handling, etc. I never had any go bad in those situations. Sour milk will in fact smell sour. You can also dab on your wrist & taste it. How clean you pump also plays a role in storage. If you are actually taking the time to sterilize, etc & wash hands & be careful, the risk of contamination is very very low. If you are a bit more lax, then you have to account for that. I was always careful the first 3 months or so & then was more relaxed as they got older. I did have a bunch of milk go bad on a business trip once & I never knew why. According to what I could recount, I had handled everything correctly, in the same manner that I always did & yet it was all bad. I have to assume I did something in introducing some bacteria either through my pump or bottles. I always traveled with bottles as they risk less leaking. All of it was stored on ice the entire trip & at no point was in out. I was only gone 4 days.
I tell you all that to tell you it was obvious it was bad. It smelled bad, it tasted awful. Honestly I think what was most upsetting is how much work I put into making sure I stored it well & all the hassle of making sure I was transporting it well & I could have just not even worried about it, because it all amounted to nothing in the end. I just dumped it a home versus there. I took trips after, did nothing different that I know of, and never had that issue again.
My kids are past this age, but the guidelines i was given when I had my 1st were what I worked with & never had issue other than the time I described. At that time they listed at 70 degrees you had 8hrs with fresh milk, in fridge (at back) you had 8 days & in standard freezer you get 6m & in deep freeze a year. I've read many that are much much more conservative, but I went with what I was given at the hospital & just taped copies of it all over my house, office, in my glove box. LOL I think now I will just never forget it after 3 yrs of reading it.
Breastmilk has a lot of natural antibiotics and antivirals and keeps lots, lots longer than pasteurized milk, formula, or most other foods. You can usually leave it out at room temperature all day - say, 8 hours - and it will still be good. You can keep it in the refrigerator for at least a week before it goes bad.
If it does go bad, you'll be able to smell it, just like pasteurized milk that goes bad. Baby will also be able to taste it and will likely refuse it.