In When should I vote to delete a question?, we only cover questions.
When should 20k users vote to delete answers?
Should this be discussed on a site-by-site basis? Why?
An answer that is not an answer should be deleted (should be a comment, doesn't answer the question etc...).
Answers that are answers but are factually wrong or are actively bad (say promote SQL Injection) should get downvoted, but not deleted.