Scripture is very clear when it comes to explaining how long God took to create the universe: ...and there was evening and there was morning, one day (Genesis 1:5). ...And there was evening and there was morning, a second day (Genesis 1:8). ...There was evening and there was morning, a third day (Genesis 1:13). ...There was evening and there was morning, a fourth day (Genesis 1:19). ...There was evening and there was morning, a fifth day (Genesis 1:23). ...God saw all that He had made, and behold, it was very good. And there was evening and there was morning, the sixth day (Genesis 1:31).
Some would advocate the the days mentioned in Genesis (all of which have both an evening and morning) are symbolic days. However, such an interpretation is faulty. While there is poetry in Scripture (in the Psalms and such), Genesis 1 certainly doesn't meet the qualifications. Moses (the author of Genesis) recorded a divinely inspired historical account. The historical account of creation in Genesis 1 is no more symbolic than the historical account that Methuselah lived 969 literal years (Genesis 5:27).
Rejecting the truth that God created the world in seven days has consequences beyond the first chapters of Genesis. For example, God gave the Israelites the ten commandments. One of these commandments concerned the sabbath. His reasoning for giving such a command was simple: For in six days the LORD made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the Lord blessed the sabbath day and made it holy (Exodus 20:11). The reasoning behind this command makes little sense if creation did not occur in seven days.
Scripture also leaves no room for the "Gap Theory". If Genesis' simple seven day timeline is too complicated, Exodus 20:11 is crystal clear. Do not attempt to mesh Christianity with lies which seek to remove God from His role as Creator of the universe. God created the world in seven days. Plain and simple. Billions of years cannot fit into seven days. It would be more honest to reject the Bible's inspiration than to knowingly misinterpret Scripture.
The question everyone should really be asking is this: Why didn't God create the world in six seconds?