The surgeon usually states how much the drainage should go down before the drains come out. That's why the doctor has probably asked you to keep track of how much fluid collects. Some people are lucky enough to get their drains out in a few days. For others it takes longer. I don't know what the average is.
Yes, you will feel some pinching, itching, and weird nerve sensations as the incision heals. However, you should be seeing an improvement, so call the doctor if the pain increases or if you see redness or other signs of infection. You can read more about common questions about mastectomy surgery at this link. I hope you are soon feeling much better.
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