EBM can be stored in a freezer for up to 3months and a deep freezer can be up to 6months.
always label the date, amount & time you have expressed it.
EBM can be stored in the fridge i think for approx 2weeks

You can buy freezer bags from chemists to put your EBM in and comes with labels (they are about $9 for a pk of 10, ive used 5 packets so far!)
and thats right you must discard any you have heated that baby does not take.
To heat it i dont think it should be microwaved but warmed in boiling water.
HTH?