As of 1 January, 2011, Serbia no longer practises mandatory military service. Prior to this, mandatory military service lasted 6 months for men. Conscientious objectors could however opt for 9 months of civil service instead.
On 15 December, 2010, the Parliament of Serbia voted to suspend mandatory military service. The decision fully came into force on January 1, 2011.
References
Conscription in Serbia Wikipedia(Text) CC BY-SA