From [here](http://meta.serverfault.com/questions/2291/deleting-closed-questions-with-no-answers) (credit to Iain) :

**If the question is more than `30` days old, and ...**

- has -1 or lower score
- has no answers
- is not locked

... it will be automatically deleted.

**If the question is more than `365` days old, and ...**

- has a score of 0 *or* a score of 1 with a deleted owner
- has no answers
- is not locked
- has a viewcount <= the age of the question in days times 1.5
- has 1 or 0 comments

... it will be automatically deleted.

OR

It will be deleted if three users with the appropriate amount of rep vote to delete it.