Thank you for your question. Without a photo or a more detailed description, it is hard to give a useful answer. If the area is small, expect a few weeks. This is a breast augmentation page, so I assume that is the operation you have had. Necrosis is rather uncommon with a simple breast enlargement operation. It is less uncommon if additional surgery, such as a breast lift was done at the same time.
Your surgeon, who knows what was done and has seen the open wound is the best source of answers to your question. Don't hesitate to call and ask--this is an appropriate question to be asking.