Some questions about pumping and storing milk?

Well I am going back to work on Saturday, but only to work 4 hours a shift because there is nowhere at my job for me to pump privately. Now my first question is how long can previously frozen milk stay in the fridge? like if I were to put the milk in the fridge over night to thaw how long would it last? My other question is if I only work one or two days a week at 8 hours without pumping in between would that mess up my supply? I ask think because I might try it out sometime so I can get more hours in at work. I have gone 8 hours at night before when my son slept without waking and I didn't really hurt at all, but he also usually sleeps at least 5 hours every night so that may have mad a difference in me filling up.