Idea taken from www.briskodesh.org
The act of creation in the forming of the fetus is completed in 40 days, and this manifests many times in scientific reality as well as in the spiritual reality. (For example, Moshe received the Torah from G-d in 40 days). When a person wishes to distance himself from a bad habit, the first forty days will always be the hardest, it is during this time that he paves a path. When a person wishes to become Shomer Habris then his hardest struggle will be during the first 40 days as he plows his path through the spirits of impurity. After the first 40 days a person has passed a large milestone and although he is far from safe, the overall test has just become easier. This does not mean that he will not be tested periodically from time to time, it means that he has made a nice separation between himself and the sin and now he can start gathering strength to fight the bigger tests when they come.
We have a member on this list who just passed the 40 day mark. He then started another round of 40 days recently, in which he pledged to try being even more careful than in the first 40 day round. Incidentally, we pointed out to him that the second 40 day period ends exactly on Shavuos, when he will be truly ready to accept the Torah. We both saw this as a sign from heaven that his hard work is being smiled upon by G-d. Today he sent us an e-mail as follows:
Well, I'm still struggling to guard my eyes, but it is getting easier though. I'm glad you are praying for me and that Hashem is having mercy on me, at least for now. What you point out about the remaining 40 days is truly amazing; in fact, [the second] 40 days is beginning to not look like such as difficult task bli ayan hara. I still need to be vigilant, now that I know the signs as a result of previous experiences....