When you write a novel.
I thought this was obvious, but my slush pile this morning turns up a book proposal for a novel...complete with "competing works" that includes non-fiction.
In case this is news to you: for a novel just send the first ten pages with a cover letter.
Proposals are for non-fiction books.
Proposals are generally used when you're selling something that hasn't been finished.
You MUST have a finished novel before querying an agent, let alone submitting it for consideration to a publisher.