I personally am of the belief that it took billions of years. O.K., my beliefs aside religious or not, in the original language the word for "day" also meant "eon".
And while billions of years seems like a long time to US... God is immortal. Time means something to us, but not something to something that has been alive all eternity and will never die. 10 billion years and 1 second would all be the same to God. Time only matters when it starts making an impact on your life, or when you have reason to notice it. Do we as humans ever notice nanoseconds? We can't, we live too long for nanoseconds to matter to us. But to something immortal, that would be time, period. Having a life span is what makes time impactful in the first place.
I always wonder what an immortal being does considering they have always been alive and never will die. In any case, I think he'd rather be disappointed with it being as short as people claim it in comparison. Even as an immortal being God would probably sense that a few billion years took a bit longer than 6 days. Maybe, I don't know since I'm not immortal. But a longer project would probably always be welcome over an extremely short project.