All business plans should be reviewed and revised at least monthly. The review should include looking for changed assumptions and analyzing plan vs actual results with management of the difference.
Like anything else in business, a business plan should be judged good or bad not in a vacuum but in its business context with its specific business objective. Most of the online discussion about business plans is focused on business plans related to seeking investment, and I`m going to make the assumption in this answer that you are asking about those.
Defining the problem you are solving for your customers is by far the most critical element of your business plan and crucial for your business success. If you can`t pinpoint a problem that your potential customers have, then you might not have a viable business concept.
Ideally, the executive summary can act as a stand-alone document that covers the highlights of your detailed plan. In fact, it`s very common for investors to ask for only the executive summary when they are evaluating your business. If they like what they see in the executive summary, they`ll often follow up with a request for a complete plan, a pitch presentation, and more in-depth financials.
Don`t bother to include terms of a potential investment, as that will always be negotiated later. Instead, just include a short statement indicating how much money you need to raise.
The most common mistake by far is on profits. Startups that grow don`t produce profits. Investors make money on valuation increases, not profits. Real businesses rarely produce more than single-digit profits. Big profit projections are sophomoric. Take all those profits and dump them into marketing expenses and you`ll be better off.