Since I have no idea what your product is, how big your team is, how much experience you have and what resources you have access to, it is hard to say how much saving is good and how much time you can spend with it (since the costs are also unkonwn).

But I would say in general you would spend about 6 months developing your product, and having something ready anywhere from a good working alpha version to something more of a working prototype and work in progress. I hardly doubt there are too many companies that can start generating revenues (let alone profits to spend on living expenses) within six months from the day they start.

So I would say the best way to do this is to start working on the idea before you quit your job, try to get a prototype or something together, and start looking for funding. I think in your case at the end of six months you will def need outside money (again since I doubt you'll be generating revenues by then and you are out of savings).

So regardless, if you do quit, I would recommend you looking for money right away. The worst thing to ever do is to ask for money when you need it, and when you are under pressure. Take your time, do your shopping, find out how interested people are in your product and try to raise some smart money before your time is up.

and please do keep us in the loop, and good luck to you