Assuming you have undergone the procedure, your plastic surgeon will always be your best resource when it comes to specific information.
Generally speaking, if there is a compromise in the blood supply to the nipple/areola complexes, it is usually evident within the first few days post op. Often, the potential diminished blood supply is even evident during surgery. Usually, once a patient is passed the first week post breast reduction or lifting surgery, and tissues appear to be healing normally, I am no longer concerned about the potential for necrosis.
I hope this helps.