From here (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.
