For any normal planning purposes, for any normal company, you should have at least 12 months detailed month by month for business plan forecasts. That would be for sales forecast, cost of sales, your burn rate, and eventually the complete financial forecast, if you’re going to do it. Then have another two years beyond that, for three years total, as annual projections.
That doesn’t mean you don’t think in longer terms. Think about what you want for your business for 5, 10, 20 years. I’m all in favor of that. But I don’t think you should plan for very long time periods in the detail of financial forecasts. The larger numbers — sales, for example, involve so much uncertainty that the time you spend trying to project more detail isn’t worth it. At least not in normal cases. If you’re farming lumber from tree farms, maybe.
Be forewarned. You’ll run into experts who will say you need more than 24 months, or more than five years in detail. They will be very sure of themselves. Sometimes what they mean is that they know more than you do, so they want you to suffer more. Or they want you to pay them to do the financials instead. Or they don’t like you or your business plan and they’re embarrassed to tell you. So instead, they say you need to forecast in more detail. If they are investors, what they mean is they don’t want to invest and they don’t want to tell you why. If they are loan managers, they don’t want to make the loan. And they don’t want to tell you the real reason.
My advice to you, when that comes up, is that unless you are a special case (if you are, you know who you are), look for another expert.