Do not try to write your business plan in one sitting. As I mentioned in the beginning, for a traditional brick and mortar business, it could take in excess of 100 hours to compile all of the information needed into a comprehensive but yet understandable document. For online businesses, probably not that long. But your final product should be well thought out, well documented and easily understandable.
I mentioned the financial aspect of a plan earlier, so let me add this. Another fact about financials to consider: not all business activities are about making money. Point being, in most enterprises financial considerations are centric to the document. But there are some other considerations. For example, a few years ago I wrote a plan for a new subsidiary that was focused on developing an inventory of patents. The potential financial returns were years into the future. Those patents may or may not ever have commercial value. Another example is a non_profit enterprise that has need for a complete roadmap for growing their profile in a market, of which a marketing plan would be the centerpiece.