It seems to me that God uses "40" whenever He is doing something that shouldn't be rushed.
40 years in the desert - the people grew very impatient, but God has His own idea about when they were ready to enter the promised land.
40 days and 40 nights of the flood - a world fallen into so much sin certainly needs a thourough cleansing.
40 days fasting in the desert - even Jesus subjected himself to something requiring great patience and perseverence.
Even now, we have something occurring all the time that helps us relate to Christ, and God's frequent use of 40 - when women conceive a child, they are pregnant for 40 weeks (for full term). This duration of time seems to take forever, with the last month certainly trying a woman's patience - it certainly did for me, twice.
Anyway, it seems 40 is used when God is not in a hurry, doing something in His own good time, making sure to be thourough - and definitely testing man's patience.
Answer by Richard Geraghty on 11/6/2007:
Thank you for the information.