Some stats from Stack Overflow
Questions with no answers:
75,422 / 1m+
Questions with no answers and score > 0:
12,705 / 1m+
Questions with no answers and score < 0:
2,016 / 1m+
Questions with no answers created more than 30 days ago:
64,717 / 1m+
Questions with no answers created more than 120 days ago:
39,915 / 1m+
Questions with no answers and score <= 0 created more than 30 days ago:
1,631 / 1m+
oldest zero answer question in the system:
http://stackoverflow.com/questions/87110
Based on this data, I am only comfortable auto-deleting the negatively voted, zero answer questions more than 30 days old. These are basically complete crap, and they're a tiny minority of questions anyway.