Let me preface this post by saying this... I think there are some very valid reasons for divorce such as abuse in the home of any kind and unfaithfulness. I have a lot of friends who were divorced for some of these reasons and God has blessed them in their second marriages with a spouse who understands what it means to have God at the center of their family. That being said, my list of valid reasons for divorce pretty much ends there. If you are struggling with anything else in your marriage and you want the quick answer to the title question of my post, my short answer is "never".
I have been married almost 10 years so trust me when I say I know how hard marriage can be. I know, I get it, I've been there. But believe me when I tell you there is hope. I don't personally know anyone who has been able to conquer a struggling marriage with anything (counseling, communicating, more sex) other than God.
God is so much bigger than the struggles in your marriage (alcoholism, addiction to pornography, trust issues, conflicts in parenting, bad tempers, lack of respect, lack of intimacy, etc.). I said this in my post from a couple of days ago but the key in fixing your marriage is making God (not your spouse) your top priority. You HAVE to give it up to Him and let Him in or you will never survive this. Trust Him. He wants your marriage to work more than anything. Give Him time. I promise He will not fail you.
If I would have given up on my marriage several years back and not trusted God and most importanty His timing, we would not have Ethan today. I can't even begin to imagine my life without him in it. My sister shared this verse with me yesterday and I have to say it could not be more true:
"Delight yourself in the Lord and He will give you the desires of your heart." ~Psalm 37:4
I think it's interesting that another bible version replaces the words "Delight yourself" with "Hope" (Hope in the Lord!).
This is one of my favorite songs by Rhett Walker Band, "When Mercy Found Me" and it reminds me of when I finally gave my control over to God and trusted Him to work in my life. I am far from perfect but He is gradually showing me day by day who He wants me to be in this life.