In short. As long as it has to be.

What to put in it? In short, a step by step instructional on how you will expose your services to the consumer.

A business plan and marketing plan are intended to be documents of reference during complex times. When you are losing your focus, gonig through self doubt - you go to these documents.

Don't spend too much time on them though. If you have an online business focus on digital marketing steps, if not then make sure to cover the position you will take in the market such as price competitiveness, value proposition, write down examples of why customers might buy in general.