I think a business plan is required almost immediately. It's just the size that differentiates. If it's for yourself, a single page or paragraph can do. It just helps founders focus on what they're trying to do. For me personally, it helps. If your looking for funding, then of coarse a larger business plan will be needed. I really think it just depends on the person or the situation. But like I said for me personally, I like to map my ideas on paper and have it easily read by others.