How long to strengthen rotator cuff?

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Ive had a few shoulder surgeries (2 left, 1 right), my right shoulder is fine but my left shoulder is weaker when the weights get heavy. I was told by doctors to do these exercises: http://en.wikipedia.org/wiki/Rotator_cuff#Strengthening
I wanted to know how long it takes to notice a difference once I start doing this or how long it will take for my left shoulder to be equal to my right?
thanks