Are you nursing? Do you need to be pumping?
Here is what I do, but I don't know if your situation is the same. I would pump for 10 minutes after each time I nursed each of my kids for the first month and then I stopped and just nursed. I wanted to make sure I built up a good supply and had some back up because I had to go back to work full time. I went back to work when she was 8 weeks old and haven't had to dip into my supply. When I am with my baby I nurse her, and then I pump 3 times (every 3 hours) while at work. She gets 3 4 oz. bottles during the day. She eats every 3 hours during the day and then sleeps 8 hours at night. If you are strictly pumping, you should do it as often as your baby eats (2-4 hours). If you're trying to get extra to put back in the freezer, you should pump for about 10 minutes after each feeding. You want to pump for at least 3 minutes after the last drop of milk. As far as getting up at night, I mentioned it to my pediatrician, and he said if I want to I could, but he said it is more imporant for me to get my rest than it is to get up and pump just to have milk in the freezer.
Oh and when I pump at work I get 2-5 oz per session. It varies every day. There are a lot of mornings that I have to pump after I nurse her to get an extra ounce or two to send to daycare. I read that getting 4 oz at once is above average. You won't get as much as your baby would get from nursing.
I know how hard it can be to chase around a toddler and try to nurse and pump (I did it during my maternity leave). If you don't have to pump, I wouldn't and just save yourself the time and the stress!
I hope this helps! Good luck!