How much is too much?

Did you ever hear about the philosophy: "fail early, fail fast". I picked up this line while reading about the "lean start-up philosophy".

Imagine a two person start-up trying to develop a web 2.0 application. Time is the most precious resource for the company and delivering a working prototype is everything! In such circumstances, the million dollar questions is: "how much time do you spend on conceptualizing, planning and designing before you start implementing the prototype? In other words, how much (time spend on design/planning) is too much?"

I know that the answer to this different for each scenario and is dependent upon - the time you have, the overall goal of the project and the immediate goal of the prototype. However I thought, it would be neat if someone could come-up with a 'COMPLEX' flowchart that helps people figure out which design methodologies are more relevant to a particular situation than others. I call it a COMPLEX flow-chart since the number of possible scenarios are infinite and each scenario warrants a unique approach. However it should be possible to identify the 10-20 most common(or general) scenarios and then create a flow chart that. For example the flowchart would tell you that - the card-sorting method is more relevant to scenario X, but usability testing with low fidelity prototypes are more suitable for scenario Y. Basically if we synthesize the different design principles, techniques, methods learned in this class into an applicability based flowchart it would be a nifty tool!

I would like to know your thoughts/opinions on this. Do you think it would be possible? Or would it be too much of a generalization? Or too complicated?