Here's a reasonable summary of what should be included.
Are you using it to get funding? In which case I would say you should try to show two full years of financial projections.
I prepare and review loads of business plans. The key really is to make them easy to read - simple font (something like arial), no jargon or technical terms. The executive summary is really important, you have to get the reader's interest in those first few paragraphs or they may not read any further.