You would first do a P&L for the present year for your existing business and the first year and as many years after as you would like to have your plan cover. Your existing business financials will be the foundation for building yourself a business plan for as many years out as you want.
Though the rate of failure is highly significant in small businesses, and it is the main reason why a good business plan is needed. Probably for raising additional cash and to provide potential investors and lenders with the information required to make investment decisions. This makes developing a business plan extremely important. One's business plan has to stand out and his projection has to be firm due to higher competition in attracting funding for his business. Investors no longer risk their money on businesses that do not prove to have great potentials for them.