Why 40 days?

So why am I devoting 40 days to this project? They say it takes three weeks to develop a new habit. Why not 21? Since I'm starting on August 1st, why not finish on August 31st, and move to the next project? The Bible mentions the number forty time and time again. When I ran a Bible search, there were over 100 results. There are several categories that I think this can be split into.

The number 40 is associated with punishment.
Noah's flood rained for 40 days and 40 nights.
God gave the Israelites to the Philistines for 40 years.
Goliath taunted the Israelite army for 40 days before David killed him.
Ninevah was given 40 days to repent after Jonah preached there.

It is also a number associated with rest.
Two times, Moses went up on the mountain and spent 40 days with God.
Several times throughout the reigns of the kings and judges, there were 40 years of peace.
David reigned for 40 years.
Solomon reigned for 40 years.
Elijah had the strength from 1 meal to go for 40 days to Mt. Horab.
After Jesus rose from the dead, He spent 40 days with his disciples before He ascended.

It was associated with mercy.
In the Old Testament, the Law said that a guilty man should be beaten, but that he could not be beaten more than 40 times.

It was a number of testing.
Jesus spent 40 days fasting and praying in the wilderness, and was there tempted by Satan.

While the number 40 is certainly prevalent throughout the Bible, it is the last reason that draws me to this time frame. If Jesus thought it was important enough to set aside 40 days to pray, meditate, and prepare himself, then that seems like a good number to me.

So, 40 days of praying, meditating on God's ideal for me, and striving to grow into that ideal it will be.