2 days is enough to cover the highlights of xian. IMO, the most recommendable site to visit in xian is the well known terracotta warriors museum. you can spare one whole day to appreciate it, generally half day is enough to visit it. it depends on your interests. as for other sites in xian, i think the big and small wild goose pagodas and the city wall are worthwhile for visiting. oh, don't forget the muslim street near the drum tower that is really popular for visitors.