Can't say how long but I do know it was ages and ages, as in months and months!
Look for a massage therapist who specialises in breast care, I saw a Remedial massage therapist recommended by my Physio - it may help if you have fluid build up that is slow to move along and just to get a little movement! I can now get my left arm up my back, like I used to but I can't bring it up and around like I used to..............there are lots of changes with muscles and nerves with a mastectomy.
Three weeks is still early for swelling and recovery
BCNA have a DVD called Strengthen Your Recovery