Answer
The hair of Nabi (sallahu Alaihi Wasallam) would at times reach up to his shoulders, thus to keep the hair up to there would be sunnah. We are unaware of any clear limit mentioned for the hair of a man, however this much is mentioned that it is not permissible for a man to imitate women. Thus a man should not allow his hair to go below his shoulders as this would be taken to be imitating women.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai