U can store at room temperature upto 6 hours . If u are using cows milk avoid adding water . Add sugar to it . U can use formula milk as well . Prepare properly The amount of milk required will of large volume as age grows and feeding frequency is also increased So what u do is u time your feeds in such a way that once u give feed just prior for leaving work and remove one for the later hours . Look into your shift timings . And adjust accordingly

Expressed breast milk can store up to 12 hours at room temperature & in refrigerator for 24 hours. Best is mother's milk but can use formula milk, cows milk advice after one year of age.