Are there any theories in the Jewish faith as to why it took God 7
days to create the earth ? (and not 1 day for example)

Hello dogsbollocks,

I have found a theory presented on a Jewish web site that bases its
explantion on the principles of the Kabbala. The Kabbala (Kabala,
Kabbalah) underpins much of Jewish philosophy and mysticism.

"Each day has its particular function (Zohar, part III, 94b)...
... G-d's creation of the universe spanned seven days, each of which
saw the creation of a new class of elements particular to the
intrinsic nature of that day. For these seven days served (and
continue to serve) as channels for the seven divine attributes
(sefirot) that the Almighty chose to invest in His creation of our
reality. The attribute of chesed, with its giving or bestowing nature,
defines the creations of the first day; the things created on the
second day embody constraint and severity, in keeping with the
attribute of gevura; and so on.... "
You can read the article at:
http://www.chabad.org/Parshah/Article.asp?AID=2788  
(Chabad.org is an organization that focuses on the use of Internet
technology to provide Jews worldwide with information about their
tradition)

If you wish to find out more about the Kabbala, there is 
an excellent introductory article by Halevi:
http://www.kabbalahsociety.org/intro/introductiontoka.html
(Some information about Halevi:
"Z'ev ben Shimon Halevi is the Hebrew name of Warren Kenton*...  
After over forty years of study...  he sees it as his task to
translate the Judaic line of Kabbalah into a modern form for any one
who wishes to walk the Way of Kabbalah.
... *NOTE: Z’ev ben Shimon Halevi born, circumcised and barmitzvahed a
Jew.  His family, on his mother’s side were Ashkenazim from Poland,
while his father was Sephardi, of a Rabbinical line, from Bessarabia -
a province in Southern Russia.  His full Hebrew name is Z’ev ben
Shimon, ben Joshua, Haham-Halevi.  Both of his families were Levites,
according to family records.  When his grandfather migrated to England
in 1900 the name Haham was recorded as Kaufman.  It was later changed
to Kenton."
http://www.kabbalahsociety.org/zbsh-int.htm )

and a series of articles, and an explanation of the 10 sefirot at:
http://www.byzant.com/kabbalah/introduction.asp  
(This site is not a Jewish one, being more linked to the Western
Hermetic tradition)

Note that the citation given above refers to 7 sefirot, while the
kabbalistic Tree of Life comprises 10 sefirot.  This is because the
first 3, sometimes the Supernal Triad, are beyond our state of
manifestation.

I hope this answers your question, but please request further
clarification if required.

Search strategy: 1. Talmud "why did it take" days create
and then, picking up on hints from the search results, I searched on:
2. zohar six days create

Geebosh
http://www.geeboosh.com/magnificent_7.htm

"In the beginning... God created 7's.

Oh sure, He created light and dark, the heavens and earth, too. But
for reasons unknown to us, He seemed to have a special affinity for
the number 7.

The fact that the Torah begins with a verse containing 7 words and 28
letters (divisible by 7) is hardly remarkable. But when placed within
the context of the overwhelming number of associations in Judaism with
'7', a fascinating tapestry begins to unfurl. Let's take a closer look
at this phenomenon."

[edit]

WHY "7"?

"Kabbalah teaches that 7 represents wholeness and completion. After 7
days, the world was complete. There are 6 directions in our world:
north, south, east, west, up and down. Add to that the place where you
are, and you have a total of 7 points of reference."

[edit]

"No one is certain why God chose the number "7" to signify completion.
All we can do is speculate, observe and marvel.

In honor of our own completion of the 49 day period leading up to
Shavuot, we present 49 allusions to the number "7" within Judaism. How
many of these do you recognize? How many more can you add?

THE MAGNIFICENT SEVENS!

1. Shabbat is the 7th day of the week.

2. There are 7 weeks in the counting of..."   etc., 

[edit]

48. We conclude our Yom Kippur prayers by proclaiming7 times, "The
Lord is God!"

49. A Jewish wedding is followed by 7 days of celebration (Sheva
Brachot).

tlspiegel

Another possible reason for it to be 7 days is to lend legitimacy to
an existing tradition of devotion on the 7th day.
See for example http://www.geocities.com/zeeshanhasan/creation.html

Also, note that on each of the days the one Jewish god is depicted
doing things that were done by multiple different gods in the gentile
creation myths.
See:
http://www.gly.uga.edu/railsback/CS/CSMarduk.html
http://members.bellatlantic.net/~vze33gpz/sumer-faq.html#A1.6
http://members.bellatlantic.net/~vze33gpz/assyrbabyl-faq.html#a1.7