Ask HN: At what point did you just start working on a prototype?

I'm ready to quit and move to another job but I started to think about a prototype for an idea I have. I've worked on it off and on, and maybe I should try to carry it as far as I reasonably can. It'd be nice to have a change of pace.

Do I just quit one day and then start work on it? Will it sound OK to employers in silicon valley if I took 3-6 months off to work on it and had nothing to show for it?