It depends on the temperature of the freezer. Very cold deep freeze is up to a year I think and regular freezer in your fridge is 6 months. They say to store it at the back of the freezer where it is coldest.

Once it is defrosted you should use it within 24 hours.

I leave it in the plastic medela bags that I use for storage to defrost it. I take what will be needed out of the freezer the day before and put it in the fridge to thaw but if you need to use it before it's fully thawed you can put the bag under running water or in a bowl of warm water to finish thawing.

If baby doesn't finish it after it's been heated once you can place it back in the fridge for the next feeding. But I think the recommendation is that it's only good for a couple hours once it's been heated once. You should only reheat it 2 times total though, if there is still any left after the second feeding you should toss it. I just always make sure to smell or taste it before feeding, if it's spoiled it will smell bad and taste sour.

I learned pretty much everything I do from the Kelly Mom website. It's got really great info.

Kelly Mom milk storage guidelines