I agree fundamentally with a lean startup approach. I think the only thing to be careful of is shortchanging yourself. I think there has to be enough put out to test and get feedback on. It will be different for every project. Some can be built quicky, while others take more time. The trick is just to identify what are the core components that make the product. That's another advantage of a lean mentality, it forces you to focus on what's important and that can help the product evolve more potently I believe.