As far as I am aware you can only stay 90 days in 180 days:
Quote:Quote:- US embassy
U.S. citizens do not need visas to stay in Serbia for up to 90 days within a 180-day period. If you wish to stay in Serbia longer than 90 days during any 180-day period, you must apply for a temporary residence permit at the local police station with authority over the place you are staying in Serbia.