Breast Milk Supply

 I had my daughter three weeks ago and I’ve been breast pumping and breast-feeding her ever since, and I’ve noticed that I haven’t been able to pump as much as I did in the first week. I noticed that even if I am alternating between my breast that I am only able to pump maybe 3 to 4 ounces on my right breast and maybe 2 ounces on my left breast. I was planning on breast-feeding for maybe a good three months and I’ve also wanted to know if anyone had any ideas to help up my milk supply. It would be helpful if you guys could let me know how many times a day should I be pumping and how many minutes long?