How long did Nefertiti rule?

Nefertiti at age 15 married Akhenaten, they both ruled during the 18th dyansty, or the New Kingdom, Akhenaten later then died, and Nefertiti did not want to rule much longer, so she steped down. How long she ruled is not clearly known but some people assume about 17-18 years, because akhenaten ruled 17.