I'm not really sticking with the day job for financial reasons though. I want to see the product I've been working on ship. Get it into the hands of actual paying customers. Because I figure then I'll get to see a whole bunch of other problems that will likely impact the startup too. Much better to make the initial mistakes on someone else's dime.
Problem is, my day job seems to move at a glacial pace. So despite having a 6 month handicap and about 1/5 the number of man-hours available, my startup may launch first. If that happens, I'll probably quit the day job whether it's done or not, because it's just a drag on the startup.
So perhaps it might be best to leave when your product is about 80% to the point of launching, because then you can focus full time on actually getting it out the door, and on the aftermath. If you have a day job, it might never seem like it's ready to launch.
But that's just my hypothesis; I don't have any actual experience with that. :)
Of course, you could argue that if one really wants their startup to succeed, they shouldn't have a day job. I'd probably even agree with you. But "when to quit" is a complicated question, particularly if you're gaining (or hopefully soon will be getting - damn boss!) valuable experience in addition to cash.