My opinion on this is just that -- mine. This is what I've found works for me, and I in no way assert that it works for anybody else.
If I intend on embarking on a side-project with an eye toward commercializing it, I quit my day job immediately. I cannot divide my attention between two serious endeavors like that, and as long as I'm thinking of, and treating, it as a "side project", I won't give it the time and attention it needs in order to have a chance of success.
I do want some money stored up for the purpose first (forever separate from my regular life savings), and if I don't have that then I'll delay starting work on the project until I do. That said, it shouldn't be a huge amount of money. Too much money gums up the works and reduces the odds of success. I want enough of a war chest to make the venture possible, but not much more. I need to be broke enough that I'm carefully considering expenditures and revenue, and to have enough risk and fear to keep driving me through the inevitable hard times. It also forces me to seek some kind of revenue stream as soon as possible, which I've found is good for the project in many, many ways.
I need to go into a venture hungry and scared.
