At which point do you quit? (dumb your idea)

Is there a point at which you feel as if you needed to quit? I am working on two projects and at times I want to quit one and focus on the other, but I do not know which one to pick. Anyways most of the time I am very very confident that I will "make something people want", but there is that 5% of doubt that hovers. When do you know you need to dumb one idea and focus on something else?