I don't think there is really an objective answer for this, but here's my take: I basically started thinking of Fogbeam Labs as a startup when I did all the legal blah, blah to create the legal entity.
When to drop everything and work on it exclusively is a different question. I still haven't done that, and me and my co-founder both still work dayjobs and hack on this by night and weekend and other "miscellaneous" time (lunch breaks, plane flights, etc.) For us, I think I'll quit my dayjob and focus on this when A. we have a paying customer definitely on the hook, ready to sign a PO, and/or B. I have enough cash reserve to give me a minimum of 6 months of runway (assuming my current $DAYJOB employer says they would want me back if it doesn't fly, which I think they would) and/or C. crowd-funding becomes a reality and we raise a reasonable but small amount of money that way.
Less likely options for us are: D. raising money from more traditional angels or VCs, or E. winning the lottery.