"Thus the heavens and the earth were finished, and all the host of them. And on the seventh day God finished his work that he had done, and he rested on the seventh day from all his work that he had done. So God blessed the seventh day and made it holy, because on it God rested from all his work that he had done in creation." Genesis 2:1-3
The first few verses of Genesis chapter two tell us what God did on the seventh day of creation. He rested. I take this to mean that God rested for a 24 hour period (count me in with AIG on this one). While I do not know what it means for God to rest, I do know that He rested for a day.
There are many who do not believe that the word "day" in Genesis chapters one and two refers to a 24 hour period. Rather, they believe it instead points to much longer periods of time, usually millions of years. The folks at Biologos, for example, think this.
If "day" in Genesis chapter one speaks of thousands or millions of years, then "day" in Genesis chapter two must mean the same. That means, according to this view, that God must have rested for thousands or millions of years. My response to this is astonishment; God must have been really tired!
We quickly see the absurdity in this. God by definition is omnipotent, sovereign, and above physical limitation. He has no need to rest for millions of years. He doesn't get tired (Jesus got tired on earth, but that was His humanity).
The word "day" is used very consistently in Genesis one-two. When interpreting this passage, we cannot simply pick-and-choose how long we think a day is. It should be the same in each instance.
If God's rest was a short one, and it certainly appears it was, then each day would have been a short period of time. Twenty-four hours sounds about right.
If there is any further question about this, the Ten Commandments make it clear how long a day is.