In CHRISTIANITY ,why the number "40 " always reoccurring??

This number , 40 has occured in almost across the Bible , beginning from Genesis , through to Revelations

1. It was recorded in Genesis that Noah built the ARK of God in 40 years ,while the flood was on for 40 days

2. Moses waited on God in the mountain for forty days before God handed the Ten commandments to him

3. **The Philistine's Goliath derided the Isrealites for 40 days before David rose to the occasion to defeat Goliath

4. Elijah ran away from the wrath of Jezebel for 40 days

5. Jesus fasted for 40 days and nights

6. Jesus return from the firmament and was seen after 40 days

7. The Isrealites wandered in the desert for 40 years

8. Kings David , Solomon and Saul reigned for 40 years

WHAT IS THE SIGNIFICANCE OF THE NUMBER "40"in Christianity??