I worked on NewsCred for 6 months while keeping my day job. I'm glad I did that - we weren't obsessed with time to market. If you are or have a product that is dependent on getting out there immediately because of 100s of competitors, then you probably should give up your day job (and find a new idea?!). But most startups can be nurtured and grown while holding a day job. You'll know when you need to give up the day job. Either you have way too much startup work or way too many users, and both cases are good.

So don't sweat it. For me, 6 months was a nice overlap. I quite my job after that and haven't looked back since.