We can know the creative "days" weren't 24 hours, even with the "evening and morning" parts, because the sun wasn't created until the third "day". Which begs the question, "evening and morning" where? Heaven maybe? How long are "days" to immortal beings? Also, since each creative "day" had its own tasks, were they even approximately the same length? Were they immediately following one another? Obviously God doesn't need daylight to see what he's doing or get a sore back or eye strain after working too long. Did he just create until he was done with certain things he wanted to do, call it a "day", then quit for a while?
Maybe the galaxies and solar systems sat idle for untold billions of years while God decided what to do with it, like a collection of parts in a guy's garage. Eventually he knows he's going to make something with them, but he's not sure what just yet. Some time later, he decided to put living creatures on one of them and watched the dinosaurs for a few million years. Like an artist honing his craft, eventually he decides to scale everything down, add some variety, then he has a eureka moment. He decides to make some of these organic life forms intelligent, which he calls the last day. Then he sits back and figures he'd see how that played out, now that these little creatures who have the ability to feel emotions and think beyond the present are here.
It's actually kind of interesting when you think about it: There was light from the first "day", but no sun or stars until the third. It appears at one time, God decided to get rid of ambient light in favor of overhead lighting.