* The northeastern United States? If so, it's difficult to give a precise length because the coastline is irregular and varies depending on how you define "coastline" (e.g., including inlets, islands, etc.).

* A specific northeastern region? For example, are you interested in the coastline of Maine, or perhaps the coastline of the entire New England region?

* A country or region in Asia or Europe? There are several regions around the world known as "Northeast".

Once you provide more information, I can try to give you a more accurate answer.