How Fast Should You Write a Book?

Lately I've read advice from self-published authors on how fast to write books. Some suggest no less than 3 novels a year. Others say you need to publish something regularly--every few weeks--so your audience stayed interested and doesn't leave. One prominent author claims to write short stories in 24 hours and then publish them.
Earlier I blogged on authors suggesting a book needs a certain word count for best effect on potential readers. 80,000-90,000 was considered ideal for novels.
The idea...