If God can just speak and something comes into existence, he can do it in a 24 hour period just as easily as a 24 billion year period. However, for the record, the word yôm is used in Genesis 1 and although it typically means a 24 hour period, it can have other interpretations as well. The reason why Bible scholars believe it means 24 hours in Genesis 1 is that elsewhere in the Bible, whenever yôm is used with a number, it means a twenty-four-hour period. Note as well that in Genesis 1, it very clearly states that for each day of creation, night followed the day…. the word day does therefore not have have an interpretation of an epoch of some sort….otherwise, there would be no need to mention the night.
Actually, Genesis 1 clearly states that Day follows Night. :)
Genesis 1:5 (KJV) And the evening and the morning were the first day.